[DEV-1965] Fix retagging.

Retagging wasn't working because the retagging view was seeing messages in an AskbotUser's User's message_set, but then trying to filter out messages directly with the AskbotUser as the key.  The AskbotUser and User id didn't match, so it failed.

More problematic was the fact that Askbot's Message was keeping a ForeignKey to User, rather than AskbotUser, so that's what I fixed, and it appears to be working now.

This is another instance where Askbot labels its model with "auth" rather than "askbot," so I had to write the migration myself - `schemamigration --auto` wouldn't detect it.
Do you know why was this needed before?  Seems weird.