[SPARK-11272][WEB UI] Add support for downloading event logs from HistoryServer UI

## What changes were proposed in this pull request?

This is a reworked PR based on feedback in #9238 after it was closed and not reopened. As suggested in that PR I've only added the download feature. This functionality already exists in the api and this allows easier access to download event logs to share with others.

I've attached a screenshot of the committed version, but I will also include alternate options with screen shots in the comments below. I'm personally not sure which option is best.

## How was this patch tested?

Manual testing

![screen shot 2016-10-07 at 6 11 12 pm](https://cloud.githubusercontent.com/assets/13952758/19209213/832fe48e-8cba-11e6-9840-749b1be4d399.png)

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

Closes #15400 from ajbozarth/spark11272.

core/src/main/resources/org/apache/spark/ui/static/historypage-template.html

@@ -59,7 +59,11 @@
           Last Updated
         </span>
       </th>
-    </tr>
+      <th>
+        <span data-toggle="tooltip" data-placement="above" title="Download the event log for this application">
+          Event Log
+        </span>
+      </th>
   </thead>
   <tbody>
   {{#applications}}
@@ -73,6 +77,7 @@
       <td><span title="{{duration}}" class="durationClass">{{duration}}</span></td>
       <td>{{sparkUser}}</td>
       <td>{{lastUpdated}}</td>
+      <td><a href="/api/v1/applications/{{id}}/{{num}}/logs" class="btn btn-info btn-mini">Download</a></td>
       {{/attempts}}
     </tr>
   {{/applications}}

core/src/main/resources/org/apache/spark/ui/static/historypage.js

@@ -133,6 +133,7 @@ $(document).ready(function() {
                         {name: 'sixth', type: "title-numeric"},
                         {name: 'seventh'},
                         {name: 'eighth'},
+                        {name: 'ninth'},
                     ],
                     "autoWidth": false,
                     "order": [[ 4, "desc" ]]