[SPARK-14245][WEB UI] Display the user in the application view

## What changes were proposed in this pull request?

The Spark UI (both active and history) should show the user who ran the application somewhere when you are in the application view. This was added under the Jobs view by total uptime and scheduler mode.

## How was this patch tested?

Manual testing

<img width="191" alt="username" src="https://cloud.githubusercontent.com/assets/13952758/14222830/6d1fe542-f82a-11e5-885f-c05ee2cdf857.png">

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

Closes #12123 from ajbozarth/spark14245.

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

@@ -80,6 +80,10 @@ private[spark] class SparkUI private (
   }
   initialize()
 
+  def getSparkUser: String = {
+    environmentListener.systemProperties.toMap.get("user.name").getOrElse("<unknown>")
+  }
+
   def getAppName: String = appName
 
   def setAppId(id: String): Unit = {

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

@@ -296,6 +296,10 @@ private[ui] class AllJobsPage(parent: JobsTab) extends WebUIPage("") {
       val summary: NodeSeq =
         <div>
           <ul class="unstyled">
+            <li>
+              <strong>User:</strong>
+              {parent.getSparkUser}
+            </li>
             <li>
               <strong>Total Uptime:</strong>
               {

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

@@ -31,6 +31,8 @@ private[ui] class JobsTab(parent: SparkUI) extends SparkUITab(parent, "jobs") {
   def isFairScheduler: Boolean =
     jobProgresslistener.schedulingMode == Some(SchedulingMode.FAIR)
 
+  def getSparkUser: String = parent.getSparkUser
+
   attachPage(new AllJobsPage(this))
   attachPage(new JobPage(this))
 }