[DISCUSS] Remove email curation queue

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.

this PR contains a data-migration to flush existing items from the email curation queue.

there'll be a follow-up PRs to [drop the email curation queue table](https://github.com/alphagov/whitehall/compare/remove-email-curation-queue-table)
For future reference, one handy trick to use in these data migrations that manipulate active record models that have gone is to simply define a bare bones class in the migration:

```ruby
class EmailCurationQueueItem < ActiveRecord
  belongs_to :edition
end
```

You can then use it as normal without having to drop down to SQL queries.