Replace User.get_profile() with dedicated model look-ups.

The user profile module is officially deprecated, and User.get_profile() is
going away in Django 1.7. Just because it's deprecated doesn't make our Profile
model obsolete, it just means we need to do the queries manually.

There's nothing in Review Board itself that causes problems here. In fact,
while I was in here I found some cases where we were using User.get_profile()
where we really wanted Profile.objects.get_or_create(), to ensure that the
profile object existed.

Testing done:
* Ran unit tests
* Smoke tested the UI

Fixes bug 3083.
Reviewed at https://reviews.reviewboard.org/r/4590/