[SPARK-26444][WEBUI] Stage color doesn't change with it's status

## What changes were proposed in this pull request?
On job page, in event timeline section, stage color doesn't change according to its status. Below are some screenshots.

ACTIVE:
<img width="550" alt="active" src="https://user-images.githubusercontent.com/12194089/50438844-c763e580-092a-11e9-84f6-6fc30e08d69b.png">
COMPLETE:
<img width="516" alt="complete" src="https://user-images.githubusercontent.com/12194089/50438847-ca5ed600-092a-11e9-9d2e-5d79807bc1ce.png">
FAILED:
<img width="325" alt="failed" src="https://user-images.githubusercontent.com/12194089/50438852-ccc13000-092a-11e9-9b6b-782b96b283b1.png">

This PR lets stage color change with it's status. The main idea is to make css style class name match the corresponding stage status.

## How was this patch tested?
Manually tested locally.

```
// active/complete stage
sc.parallelize(1 to 3, 3).map { n => Thread.sleep(10* 1000); n }.count
// failed stage
sc.parallelize(1 to 3, 3).map { n => Thread.sleep(10* 1000); throw new Exception() }.count
```

Note we need to clear browser cache to let new `timeline-view.css` take effect. Below are screenshots after this PR.

ACTIVE:
<img width="569" alt="active-after" src="https://user-images.githubusercontent.com/12194089/50439986-08f68f80-092f-11e9-85d9-be1c31aed13b.png">
COMPLETE:
<img width="567" alt="complete-after" src="https://user-images.githubusercontent.com/12194089/50439990-0bf18000-092f-11e9-8624-723958906e90.png">
FAILED:
<img width="352" alt="failed-after" src="https://user-images.githubusercontent.com/12194089/50439993-101d9d80-092f-11e9-8dfd-3e20536f2fa5.png">

Closes #23385 from seancxmao/timeline-stage-color.

Authored-by: seancxmao <seancxmao@gmail.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

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

@@ -98,12 +98,12 @@ rect.getting-result-time-proportion {
   cursor: pointer;
 }
 
-.vis-timeline .vis-item.stage.succeeded {
+.vis-timeline .vis-item.stage.complete {
   background-color: #A0DFFF;
   border-color: #3EC0FF;
 }
 
-.vis-timeline .vis-item.stage.succeeded.vis-selected {
+.vis-timeline .vis-item.stage.complete.vis-selected {
   background-color: #A0DFFF;
   border-color: #3EC0FF;
   z-index: auto;
@@ -130,12 +130,12 @@ rect.getting-result-time-proportion {
   stroke: #FF4D6D;
 }
 
-.vis-timeline .vis-item.stage.running {
+.vis-timeline .vis-item.stage.active {
   background-color: #A2FCC0;
   border-color: #36F572;
 }
 
-.vis-timeline .vis-item.stage.running.vis-selected {
+.vis-timeline .vis-item.stage.active.vis-selected {
   background-color: #A2FCC0;
   border-color: #36F572;
   z-index: auto;

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

@@ -62,7 +62,7 @@ private[ui] class JobPage(parent: JobsTab, store: AppStatusStore) extends WebUIP
       val stageId = stage.stageId
       val attemptId = stage.attemptId
       val name = stage.name
-      val status = stage.status.toString
+      val status = stage.status.toString.toLowerCase(Locale.ROOT)
       val submissionTime = stage.submissionTime.get.getTime()
       val completionTime = stage.completionTime.map(_.getTime())
         .getOrElse(System.currentTimeMillis())