[SPARK-18837][WEBUI] Very long stage descriptions do not wrap in the UI

## What changes were proposed in this pull request?

This issue was reported by wangyum.

In the AllJobsPage, JobPage and StagePage, the description length was limited before like as follows.

![ui-2 0 0](https://cloud.githubusercontent.com/assets/4736016/21319673/8b225246-c651-11e6-9041-4fcdd04f4dec.gif)

But recently, the limitation seems to have been accidentally removed.

![ui-2 1 0](https://cloud.githubusercontent.com/assets/4736016/21319825/104779f6-c652-11e6-8bfa-dfd800396352.gif)

The cause is that some tables are no longer `sortable` class although they were, and `sortable` class does not only mark tables as sortable but also limited the width of their child `td` elements.
The reason why now some tables are not `sortable` class is because another sortable mechanism was introduced by #13620 and #13708 with pagination feature.

To fix this issue, I've introduced new class `table-cell-width-limited` which limits the description cell width and the description is like what it was.

<img width="1260" alt="2016-12-20 1 00 34" src="https://cloud.githubusercontent.com/assets/4736016/21320478/89141c7a-c654-11e6-8494-f8f91325980b.png">

## How was this patch tested?

Tested manually with my browser.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #16338 from sarutak/SPARK-18837.

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -246,4 +246,8 @@ a.expandbutton {
   text-align: center;
   margin: 0;
   padding: 4px 0;
+}
+
+.table-cell-width-limited td {
+  max-width: 600px;
 }
\ No newline at end of file

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

@@ -500,7 +500,8 @@ private[ui] class JobPagedTable(
   override def tableId: String = jobTag + "-table"
 
   override def tableCssClass: String =
-    "table table-bordered table-condensed table-striped table-head-clickable"
+    "table table-bordered table-condensed table-striped " +
+      "table-head-clickable table-cell-width-limited"
 
   override def pageSizeFormField: String = jobTag + ".pageSize"
 

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

@@ -149,7 +149,8 @@ private[ui] class StagePagedTable(
   override def tableId: String = stageTag + "-table"
 
   override def tableCssClass: String =
-    "table table-bordered table-condensed table-striped table-head-clickable"
+    "table table-bordered table-condensed table-striped " +
+      "table-head-clickable table-cell-width-limited"
 
   override def pageSizeFormField: String = stageTag + ".pageSize"