[DEV-1320] Get Askbot running alongside Rover

DEV-1320 contains all changes that were required to get Rover and Askbot running as part of the same Django project.  Included are the AskbotUser customizations, 'askbot_' prefixes added to Askbot table names that were missing them, timezone fixes, and a stripped-down context processor whose code was moved to a Rover context processor.

Task DEV-1320 is now tentatively done: Askbot and Rover can run as part of the same project with no immediately apparent problems.  Any further work will be specific to customizing Askbot for Rover, and so will no longer be merged into branch DEV-1320, but instead into askrover.