Remove email curation queue feature

www.agileplannerapp.com/boards/173808/cards/6034

As a DCLG editor
I want documents tagged to "relevant to local government"
  to not be held in a curation queue
So that I no longer have to curate email alerts for
  local government users

instead these notifications will be sent to govdelivery
as soon as documents are published.
Data migration to flush email curation queue

www.agileplannerapp.com/boards/173808/cards/6034

before we remove the email curation queue feature
we need to send notifications to subscribers for
any pending items in that queue. this migration
takes care of that.
Merge pull request #1821 from alphagov/remove-email-curation-queue

Remove email curation queue