[SPARK-13775] History page sorted by completed time desc by default.

## What changes were proposed in this pull request?
Originally the page is sorted by AppID by default.
After tests with users' feedback, we think it might be best to sort by completed time (desc).

## How was this patch tested?
Manually test, with screenshot as follows.
![sorted-by-complete-time-desc](https://cloud.githubusercontent.com/assets/11683054/13647686/d6dea924-e5fa-11e5-8fc5-68e039b74b6f.png)

Author: zhuol <[email protected]>

Closes apache#11608 from zhuoliu/13775.

Author: zhuol

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

@@ -165,7 +165,7 @@ $(document).ready(function() {
                         {name: 'eighth'},
                     ],
                     "autoWidth": false,
-                    "order": [[ 0, "desc" ]]
+                    "order": [[ 4, "desc" ]]
         };
 
         var rowGroupConf = {