Deleting document deletes collection memberships

www.agileplannerapp.com/boards/173808/cards/8071
resolves: https://govuk.zendesk.com/tickets/866860

a document collection cannot be re-editioned if its groups contain deleted documents. if we destroy all `DocumentCollectionGroupMembership` when a `Document` is destroyed, this issue will not occur, and will also make sure references to documents are valid.

these deleted document memberships don't surface up in admin or on the public site because we use [`DocumentCollectionGroup#latest_editions`](https://github.com/alphagov/whitehall/blob/master/app/views/admin/document_collection_groups/index.html.erb#L72) and [`DocumentCollectionGroup#published_editions`](https://github.com/alphagov/whitehall/blob/master/app/controllers/document_collections_controller.rb#L7) which filter out these dangling records.

also includes a data migration to delete ~230 dangling `DocumentCollectionGroupMembership` records referring to deleted documents, which were created in absence of `dependent: delete_all`.