Wrong URL insert in database for review usr change

The wrong URL was inserted in database for user change in reviews (added or deleted), when the root is not / but a subfolder.
The Django function that was used to retrieve the URL get_absolute_url() for a particular user was returning the wrong url. So we now have a new function that generates the right URL from the local_site_name and username.

Tested with a different SITE_ROOT parameter. (e.g: SITE_ROOT = '/reviewboard/' instead of SITE_ROOT = '/')