Cookbook deletion notification

:fork_and_knife: 

This adds deletion emails that get sent out when a cookbook is deleted.  You'll note that I specifically disabled `dependent: :destroy` for cookbook followers because while we do want these relationships to be deleted, we want them to persist around long enough for us to find out which users they're attached to and send those people emails.  The alternative to this approach was a "marked for deletion" flag of sorts, which @bcobb and I both decided was pretty gross.