[SPARK-8702] [WEBUI] Avoid massive concating strings in Javascript

When there are massive tasks, such as `sc.parallelize(1 to 100000, 10000).count()`, the generated JS codes have a lot of string concatenations in the stage page, nearly 40 string concatenations for one task.

We can generate the whole string for a task instead of execution string concatenations in the browser.

Before this patch, the load time of the page is about 21 seconds.
![screen shot 2015-06-29 at 6 44 04 pm](https://cloud.githubusercontent.com/assets/1000778/8406644/eb55ed18-1e90-11e5-9ad5-50d27ad1dff1.png)

After this patch, it reduces to about 17 seconds.

![screen shot 2015-06-29 at 6 47 34 pm](https://cloud.githubusercontent.com/assets/1000778/8406665/087003ca-1e91-11e5-80a8-3485aa9adafa.png)

One disadvantage is that the generated JS codes become hard to read.

Author: zsxwing <zsxwing@gmail.com>

Closes #7082 from zsxwing/js-string and squashes the following commits:

b29231d [zsxwing] Avoid massive concating strings in Javascript

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -572,55 +572,55 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
         val attempt = taskInfo.attempt
         val timelineObject =
           s"""
-             {
-               'className': 'task task-assignment-timeline-object',
-               'group': '$executorId',
-               'content': '<div class="task-assignment-timeline-content"' +
-                 'data-toggle="tooltip" data-placement="top"' +
-                 'data-html="true" data-container="body"' +
-                 'data-title="${s"Task " + index + " (attempt " + attempt + ")"}<br>' +
-                 'Status: ${taskInfo.status}<br>' +
-                 'Launch Time: ${UIUtils.formatDate(new Date(launchTime))}' +
-                 '${
+             |{
+               |'className': 'task task-assignment-timeline-object',
+               |'group': '$executorId',
+               |'content': '<div class="task-assignment-timeline-content"
+                 |data-toggle="tooltip" data-placement="top"
+                 |data-html="true" data-container="body"
+                 |data-title="${s"Task " + index + " (attempt " + attempt + ")"}<br>
+                 |Status: ${taskInfo.status}<br>
+                 |Launch Time: ${UIUtils.formatDate(new Date(launchTime))}
+                 |${
                      if (!taskInfo.running) {
                        s"""<br>Finish Time: ${UIUtils.formatDate(new Date(finishTime))}"""
                      } else {
                         ""
                       }
-                   }' +
-                 '<br>Scheduler Delay: $schedulerDelay ms' +
-                 '<br>Task Deserialization Time: ${UIUtils.formatDuration(deserializationTime)}' +
-                 '<br>Shuffle Read Time: ${UIUtils.formatDuration(shuffleReadTime)}' +
-                 '<br>Executor Computing Time: ${UIUtils.formatDuration(executorComputingTime)}' +
-                 '<br>Shuffle Write Time: ${UIUtils.formatDuration(shuffleWriteTime)}' +
-                 '<br>Result Serialization Time: ${UIUtils.formatDuration(serializationTime)}' +
-                 '<br>Getting Result Time: ${UIUtils.formatDuration(gettingResultTime)}">' +
-                 '<svg class="task-assignment-timeline-duration-bar">' +
-                 '<rect class="scheduler-delay-proportion" ' +
-                   'x="$schedulerDelayProportionPos%" y="0px" height="26px"' +
-                   'width="$schedulerDelayProportion%""></rect>' +
-                 '<rect class="deserialization-time-proportion" '+
-                   'x="$deserializationTimeProportionPos%" y="0px" height="26px"' +
-                   'width="$deserializationTimeProportion%"></rect>' +
-                 '<rect class="shuffle-read-time-proportion" ' +
-                   'x="$shuffleReadTimeProportionPos%" y="0px" height="26px"' +
-                   'width="$shuffleReadTimeProportion%"></rect>' +
-                 '<rect class="executor-runtime-proportion" ' +
-                   'x="$executorRuntimeProportionPos%" y="0px" height="26px"' +
-                   'width="$executorComputingTimeProportion%"></rect>' +
-                 '<rect class="shuffle-write-time-proportion" ' +
-                   'x="$shuffleWriteTimeProportionPos%" y="0px" height="26px"' +
-                   'width="$shuffleWriteTimeProportion%"></rect>' +
-                 '<rect class="serialization-time-proportion" ' +
-                   'x="$serializationTimeProportionPos%" y="0px" height="26px"' +
-                   'width="$serializationTimeProportion%"></rect>' +
-                 '<rect class="getting-result-time-proportion" ' +
-                   'x="$gettingResultTimeProportionPos%" y="0px" height="26px"' +
-                   'width="$gettingResultTimeProportion%"></rect></svg>',
-               'start': new Date($launchTime),
-               'end': new Date($finishTime)
-             }
-           """
+                   }
+                 |<br>Scheduler Delay: $schedulerDelay ms
+                 |<br>Task Deserialization Time: ${UIUtils.formatDuration(deserializationTime)}
+                 |<br>Shuffle Read Time: ${UIUtils.formatDuration(shuffleReadTime)}
+                 |<br>Executor Computing Time: ${UIUtils.formatDuration(executorComputingTime)}
+                 |<br>Shuffle Write Time: ${UIUtils.formatDuration(shuffleWriteTime)}
+                 |<br>Result Serialization Time: ${UIUtils.formatDuration(serializationTime)}
+                 |<br>Getting Result Time: ${UIUtils.formatDuration(gettingResultTime)}">
+                 |<svg class="task-assignment-timeline-duration-bar">
+                 |<rect class="scheduler-delay-proportion"
+                   |x="$schedulerDelayProportionPos%" y="0px" height="26px"
+                   |width="$schedulerDelayProportion%""></rect>
+                 |<rect class="deserialization-time-proportion"
+                   |x="$deserializationTimeProportionPos%" y="0px" height="26px"
+                   |width="$deserializationTimeProportion%"></rect>
+                 |<rect class="shuffle-read-time-proportion"
+                   |x="$shuffleReadTimeProportionPos%" y="0px" height="26px"
+                   |width="$shuffleReadTimeProportion%"></rect>
+                 |<rect class="executor-runtime-proportion"
+                   |x="$executorRuntimeProportionPos%" y="0px" height="26px"
+                   |width="$executorComputingTimeProportion%"></rect>
+                 |<rect class="shuffle-write-time-proportion"
+                   |x="$shuffleWriteTimeProportionPos%" y="0px" height="26px"
+                   |width="$shuffleWriteTimeProportion%"></rect>
+                 |<rect class="serialization-time-proportion"
+                   |x="$serializationTimeProportionPos%" y="0px" height="26px"
+                   |width="$serializationTimeProportion%"></rect>
+                 |<rect class="getting-result-time-proportion"
+                   |x="$gettingResultTimeProportionPos%" y="0px" height="26px"
+                   |width="$gettingResultTimeProportion%"></rect></svg>',
+               |'start': new Date($launchTime),
+               |'end': new Date($finishTime)
+             |}
+           |""".stripMargin.replaceAll("\n", " ")
         timelineObject
       }.mkString("[", ",", "]")