[SPARK-16808][CORE] History Server main page does not honor APPLICATION_WEB_PROXY_BASE

## What changes were proposed in this pull request?

Application links generated on the history server UI no longer (regression from 1.6) contain the configured spark.ui.proxyBase in the links. To address this, made the uiRoot available globally to all javascripts for Web UI. Updated the mustache template (historypage-template.html) to include the uiroot for rendering links to the applications.

The existing test was not sufficient to verify the scenario where ajax call is used to populate the application listing template, so added a new selenium test case to cover this scenario.

## How was this patch tested?

Existing tests and a new unit test.
No visual changes to the UI.

Author: Vinayak <vijoshi5@in.ibm.com>

Closes #15742 from vijoshi/SPARK-16808_master.