Use HTML Relative Links to Access UI Behind Gateways

The Spark UI should be accessible behind a gateway that redirects based on the URL path (i.e. http://domain.com/spark --> http://private.domain.com:8080).

Currently the Spark UI uses root links in the html page. It would be nice if it used relative links instead. By eliminating the first forward-slash "/" the Spark UI can be accessible behind an HTTP gateway.

For example, in core/src/main/scala/org/apache/spark/ui/UIUtils.scala

...
<link rel="stylesheet" href=

Unknown macro: {prependBaseUri("/static/bootstrap.min.css")}

type="text/css"/>
...

could be changed to

...
<link rel="stylesheet" href=

Unknown macro: {prependBaseUri("static/bootstrap.min.css")}

type="text/css"/>
...

This would allow the Spark UI to be accessible behind a gateway that redirects based on the URL path.