[#102] Mail private topic users

* Update griddler and devise
* Add new necessary field to user model for devise upgrade
* Add topic mailer
* Add topics observer
* Update at notification emails
* Split notifications into separate concerns. One for @'s and one for private topics
* Various tests needed updating
* Fix migrations. mv away from devise helpers. Previous versions of devise
  had migration helper methods instead of just requiring you to create the
  columns themselves. New versions of devise deprecate that so we have to
  go back and update old migrations to not use those helpers
* Add notified members to post's list