Add email notifications for proposal updates

* Add fairly awfully named WatchNotification model to track who wants
  emails about what
* New can(:watch, :proposal) feature flag so you can easily get rid of
  the emailing stuff if you don't want it
* Add controller and view stuff for emailing in proposals_controller
* Option to get email notifications on your first suggestion on someone
  else's proposal
* Creating a proposal watches it for you
* Add Mailers
* Add Mandrill config aimed at Heroku