Add ability to deactivate user accounts #129

Admins can be associated to trackable tag events. As a result, trying to delete an admin after they've logged events causes an (expected) constraint error:

Cannot delete or update a parent row: a foreign key constraint fails (brickhack-prod.trackable_events...

We definitely don't want to end up with a database of broken associations.

Instead, I think the best solution to this would be to add the ability to deactivate a user's account, disallowing them from signing in.

This StackOverflow post has some good context on how Devise can easily support this: https://stackoverflow.com/questions/6004216/devise-how-do-i-forbid-certain-users-from-signing-in