Celery - Long Running Jobs & Scheduled Tasks

[galador]

There should be some support for performing "batch" or long-running jobs. We should be able to use Celery for this.

Some examples of things that would need a long-running task for:

• User promotion / demotion.
• Sending out invites (at least, we don't send out invites immediately, they get queued up and a batch gets sent out at once on a pseudo-random schedule)
• Giving out invites to User Classes that have earned them
• Purge old uploads
• Disable old user accounts
• Expire warning / bans

[jzelinskie]

Applying updates from the tracker to the database is another potential task.