[SPARK-21675][WEBUI] Add a navigation bar at the bottom of the Details for Stage Page

## What changes were proposed in this pull request?

1. In Spark Web UI, the Details for Stage Page don't have a navigation bar at the bottom. When we drop down to the bottom, it is better for us to see a navi bar right there to go wherever we what.
2. Executor ID is not equivalent to Host, it may be  better to separate them, and then we can group the tasks by Hosts .

## How was this patch tested?
manually test
![wx20170809-165606](https://user-images.githubusercontent.com/8326978/29114161-f82b4920-7d25-11e7-8d0c-0c036b008a78.png)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Kent Yao <yaooqinn@hotmail.com>

Closes #18893 from yaooqinn/SPARK-21675.

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

@@ -94,14 +94,16 @@ private[ui] trait PagedTable[T] {
     val _dataSource = dataSource
     try {
       val PageData(totalPages, data) = _dataSource.pageData(page)
+      val pageNavi = pageNavigation(page, _dataSource.pageSize, totalPages)
       <div>
-        {pageNavigation(page, _dataSource.pageSize, totalPages)}
+        {pageNavi}
         <table class={tableCssClass} id={tableId}>
           {headers}
           <tbody>
             {data.map(row)}
           </tbody>
         </table>
+        {pageNavi}
       </div>
     } catch {
       case e: IndexOutOfBoundsException =>

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

@@ -835,7 +835,8 @@ private[ui] class TaskTableRowData(
     val speculative: Boolean,
     val status: String,
     val taskLocality: String,
-    val executorIdAndHost: String,
+    val executorId: String,
+    val host: String,
     val launchTime: Long,
     val duration: Long,
     val formatDuration: String,
@@ -1017,7 +1018,8 @@ private[ui] class TaskDataSource(
       info.speculative,
       info.status,
       info.taskLocality.toString,
-      s"${info.executorId} / ${info.host}",
+      info.executorId,
+      info.host,
       info.launchTime,
       duration,
       formatDuration,
@@ -1047,7 +1049,8 @@ private[ui] class TaskDataSource(
       case "Attempt" => Ordering.by(_.attempt)
       case "Status" => Ordering.by(_.status)
       case "Locality Level" => Ordering.by(_.taskLocality)
-      case "Executor ID / Host" => Ordering.by(_.executorIdAndHost)
+      case "Executor ID" => Ordering.by(_.executorId)
+      case "Host" => Ordering.by(_.host)
       case "Launch Time" => Ordering.by(_.launchTime)
       case "Duration" => Ordering.by(_.duration)
       case "Scheduler Delay" => Ordering.by(_.schedulerDelay)
@@ -1200,7 +1203,7 @@ private[ui] class TaskPagedTable(
     val taskHeadersAndCssClasses: Seq[(String, String)] =
       Seq(
         ("Index", ""), ("ID", ""), ("Attempt", ""), ("Status", ""), ("Locality Level", ""),
-        ("Executor ID / Host", ""), ("Launch Time", ""), ("Duration", ""),
+        ("Executor ID", ""), ("Host", ""), ("Launch Time", ""), ("Duration", ""),
         ("Scheduler Delay", TaskDetailsClassNames.SCHEDULER_DELAY),
         ("Task Deserialization Time", TaskDetailsClassNames.TASK_DESERIALIZATION_TIME),
         ("GC Time", ""),
@@ -1271,8 +1274,9 @@ private[ui] class TaskPagedTable(
       <td>{if (task.speculative) s"${task.attempt} (speculative)" else task.attempt.toString}</td>
       <td>{task.status}</td>
       <td>{task.taskLocality}</td>
+      <td>{task.executorId}</td>
       <td>
-        <div style="float: left">{task.executorIdAndHost}</div>
+        <div style="float: left">{task.host}</div>
         <div style="float: right">
         {
           task.logs.map {