Add support for extensions in UserPage
    
This patch adds support for adding extensions in UserPage and works very similar to the DashboardHook. It takes a dictionary of label, url and an optional subitems -- which itself can be an array of (label,url) -- to add custom links in the /users/<username> page.

Also updated the documentation page to indicate the usage of this. Im not sure how to build the doc page into html, so the formatting hasnt been checked for the new documentation.

I added support for this hook so that people wanting to build user-specific reports can use this. For e.g. reports like review-requests assigned to me, or a users' RSS feed can all be linked directly from a user page.

Ran all existing tests. 
Added a new test for the UserPageHook.
Added a new test extension that demonstrates the use of this (Screenshot attached).
Tested without extension to ensure no other breakage.