[SPARK-21155][WEBUI] Add (? running tasks) into Spark UI progress

## What changes were proposed in this pull request?

Add metric on number of running tasks to status bar on Jobs / Active Jobs.

## How was this patch tested?

Run a long running (1 minute) query in spark-shell and use localhost:4040 web UI to observe progress.  See jira for screen snapshot.

Author: Eric Vandenberg <ericvandenberg@fb.com>

Closes #18369 from ericvandenbergfb/runningTasks.

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -356,6 +356,7 @@ private[spark] object UIUtils extends Logging {
     <div class="progress">
       <span style="text-align:center; position:absolute; width:100%; left:0;">
         {completed}/{total}
+        { if (failed == 0 && skipped == 0 && started > 0) s"($started running)" }
         { if (failed > 0) s"($failed failed)" }
         { if (skipped > 0) s"($skipped skipped)" }
         { reasonToNumKilled.toSeq.sortBy(-_._2).map {