Remove email curation queue

This change has already been [approved earlier](https://github.com/alphagov/whitehall/pull/1761).

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 PR to [drop the email curation queue table](https://github.com/alphagov/whitehall/compare/remove-email-curation-queue-table) once we're happy with this on production