[DEV-1965] Change message ForeignKey from auth User to AskbotUser.
[DEV-1965] Add migration for ForeignKey change.
[DEV-1965] Fix migration script.
[DEV-1965] Fix RelatedObjectSimulator to point at AskbotUser.
[DEV-1965] Remove fake 'message_set' in favor of real one.
[DEV-1965] Remove comment.
[DEV-1965] Update migrations to alter existing messages.
[DEV-1965] Fix 'drop_foreign_key' to 'delete_foreign_key' in migrations.
[DEV-1965] Update migration logic to use id fields (so it's not confounded by the deletion of the ForeignKey).
[DEV-1965] Use ORM calls over imported models.
Merge pull request #36 from roverdotcom/DEV-1965

[DEV-1965] Fix retagging.