Email curation queue

Tracker: https://www.pivotaltracker.com/story/show/48066725

Local government content instead of going direct to gov delivery is instead placed on a queue that admin users can manage.  They have the option of deleting or sending the notification, and can edit the title and summary that will be used in the email.

We update the email to always include the summary even if a change note is present (this gives parity with the atom feed).  Curators cannot edit the change note, or remove it.

The Edition::GovUkDelivery code has refactored heavily to push all the main logic out of the Edition and into objects that know what they are doing.  This will break the existing work on #422 but it shouldn't be too hard to redo (a rebase might be a bit of a nightmare).

Currently this is open to all, need to know who should be allowed to do it so we can lock it down (perhaps this is a new role / permission?).