[SPARK-4310][WebUI] Sort 'Submitted' column in Stage page by time

Author: zsxwing <[email protected]>

Closes #3179 from zsxwing/SPARK-4310 and squashes the following commits:

b0d29f5 [zsxwing] Sort 'Submitted' column in Stage page by time

(cherry picked from commit 825709a)
Signed-off-by: Andrew Or <[email protected]>

Author: zsxwing

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

@@ -175,7 +175,9 @@ private[ui] class StageTableBase(
       Seq.empty
     }} ++
     <td>{makeDescription(s)}</td>
-    <td valign="middle">{submissionTime}</td>
+    <td sorttable_customkey={s.submissionTime.getOrElse(0).toString} valign="middle">
+      {submissionTime}
+    </td>
     <td sorttable_customkey={duration.getOrElse(-1).toString}>{formattedDuration}</td>
     <td class="progress-cell">
       {makeProgressBar(stageData.numActiveTasks, stageData.completedIndices.size,