[SPARK-23644][CORE][UI] Use absolute path for REST call in SHS

## What changes were proposed in this pull request?

SHS is using a relative path for the REST API call to get the list of the application is a relative path call. In case of the SHS being consumed through a proxy, it can be an issue if the path doesn't end with a "/".

Therefore, we should use an absolute path for the REST call as it is done for all the other resources.

## How was this patch tested?

manual tests
Before the change:
![screen shot 2018-03-10 at 4 22 02 pm](https://user-images.githubusercontent.com/8821783/37244190-8ccf9d40-2485-11e8-8fa9-345bc81472fc.png)

After the change:
![screen shot 2018-03-10 at 4 36 34 pm 1](https://user-images.githubusercontent.com/8821783/37244201-a1922810-2485-11e8-8856-eeab2bf5e180.png)

Author: Marco Gaido <[email protected]>

Closes apache#20794 from mgaido91/SPARK-23644.

Author: mgaido91

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

@@ -113,7 +113,7 @@ $(document).ready(function() {
       status: (requestedIncomplete ? "running" : "completed")
     };
 
-    $.getJSON("api/v1/applications", appParams, function(response,status,jqXHR) {
+    $.getJSON(uiRoot + "/api/v1/applications", appParams, function(response,status,jqXHR) {
       var array = [];
       var hasMultipleAttempts = false;
       for (i in response) {
@@ -151,7 +151,7 @@ $(document).ready(function() {
         "showCompletedColumns": !requestedIncomplete,
       }
 
-      $.get("static/historypage-template.html", function(template) {
+      $.get(uiRoot + "/static/historypage-template.html", function(template) {
         var sibling = historySummary.prev();
         historySummary.detach();
         var apps = $(Mustache.render($(template).filter("#history-summary-template").html(),data));