[DEV-1610] Disable post_save signal handler for creating new AskbotUsers.
[DEV-1610] Pass auth User into Django Group machinery instead of AskbotUser.
[DEV-1610] Alter AuthUserGroup model with foreign key to AuthUser.
[DEV-1610] Replace migrations with a migration with corrected ORM freeze for auth_user_groups (should have no effect).
[DEV-1610] Replace datetime in migration with datetime_utils from South.
[DEV-1610] Tweak Askbot's Group & related models to use auth User internally.
[DEV-1610] Tweak AskbotUser to present its underlying auth User to Groups, where needed.
[DEV-1610] Pass AskbotUser.user into Group method.
[DEV-1610] Examine askbot_user in moderate_group_joining instead of user.
[DEV-1610] Explicitly type incoming user arg to Group.__init__.
[DEV-1610] Correct AskbotUser to User in Group.__init__.
[askrover] Merge branch 'DEV-1610' into askrover

Conflicts:
	askbot/models/askbot_user.py