We had two problems that were potentially causing the existence of duplicate `LocalSiteProfile` models. The first of these is that this wasn't using `ConcurrencyManager`, which handles integrity issues across separate threads (or servers). The second, more difficult one, is that some databases (particularly MySQL) were ignoring our unique_together constraint when one of the relations was NULL. This meant it was possible to end up with multiple `LocalSiteProfile` objects with `local_site=None`. This change moves us over to `ConcurrencyManager`, and implements a check for the duplicate case when attempting to use `User.get_site_profile`. If we do find duplicates, they'll be consolidated into a single one before returning. Testing Done: Ran unit tests.