Make "spark logo" link refer to "/".

This is not an issue with the driver UI, but when you fire
up the history server, there's currently no way to go back to
the app listing page without editing the browser's location
field (since the logo's link points to the root of the
application's own UI - i.e. the "stages" tab).

The change just points the logo link to "/", which is the app
listing for the history server, and the stages tab for the
driver's UI.