Reverting Registration email changes.

This is basically this last week's worth of commits without my changes to the activation email process. ~~Up until fc3d20a041, I can get tests to pass. As soon as I add bc38c89d, tests fail. I experienced similar failures by cloning a fresh merge branch and trying to do a `syncdb` with an empty database. Rolling back those two commits allows tests to work again for me. Not sure what's going on there.~~ celery's API changed (used to return a Bool where it now returns a callable in some places). 

I'm running tests now and will update once I finish.

**note**: Merging this will reopen [PD-312] and [PD-221]. [PD-423] was caused by these changes and would be resolved by [PD-444], so those two can be closed as well.