The grand Updates refactor

Changed the way that updates work, including:
* Renaming alerts to updates
* Making updates automatic
* Moving email notifications to SQL driven templates per update type
* Implementing attendee updates
* Moving updates to the controllers to accommodate future batch processes (import)
* Fewer SQL queries (for things like the "unread" flag)
* Drop the mentions table (as updates table deprecates it)
* Drop superfluous and confusing columns on the updates table
* Adding indexes to item_type_id and item_id pairings
* Sped up the permissions queries
* More logical layout of code