Fix Broken User Links when SITE_ROOT != '/'

When Reviewboard is installed *not* at the root of a server, say, /reviewboard/,
the user links are broken. This happens in many places where User Links are
obtained, for e.g. a) when searching, b) change description links when a user is
changed in the Review Request.

The application tries to use Django's User.get_absolute_url(), which always
returns /users/<username> ignoring the application path.
This patch fixes this in 2 places
a) Adding a serialize_url_field to UserResource. This would fix all API calls
b) Adding a ABSOLUTE_URL_OVERRIDES section in settings.py for auth.user.
   (https://docs.djangoproject.com/en/1.4/ref/settings/#absolute-url-overrides)