[SPARK-13275][WEB UI] Visually clarified executors start time in timeline

## What changes were proposed in this pull request?

Updated the Executors added/removed bubble in the time line so it's clearer where it starts. Now the bubble is left justified on the start time (still also denoted by the line) rather than center justified.

## How was this patch tested?

Manually tested UI

<img width="596" alt="screen shot 2016-10-17 at 6 04 36 pm" src="https://cloud.githubusercontent.com/assets/13952758/19496563/e6c9186e-953c-11e6-85e4-63309a553f65.png">
<img width="492" alt="screen shot 2016-10-17 at 5 54 09 pm" src="https://cloud.githubusercontent.com/assets/13952758/19496568/e9f06132-953c-11e6-8901-54405ebc7f5b.png">

Author: Alex Bozarth <ajbozart@us.ibm.com>

Closes #15536 from ajbozarth/spark13275.

core/src/main/resources/org/apache/spark/ui/static/timeline-view.js

@@ -24,6 +24,7 @@ function drawApplicationTimeline(groupArray, eventObjArray, startTime, offset) {
       return a.value - b.value
     },
     editable: false,
+    align: 'left',
     showCurrentTime: false,
     min: startTime,
     zoomable: false,
@@ -99,6 +100,7 @@ function drawJobTimeline(groupArray, eventObjArray, startTime, offset) {
       return a.value - b.value;
     },
     editable: false,
+    align: 'left',
     showCurrentTime: false,
     min: startTime,
     zoomable: false,