Only show specialist sectors field when content API available

If the content API is unavailable the field isn't rendered which allows
editors to continue with their work as normal. Any specialist sector
associations on the drafts being saved will remain unchanged.
Fix typo in specialist sector scenario
Add dependent destroy to MainstreamCategory joins

When a category is destroyed all the join records should also be
removed.

Amend the oil and gas specialist sector data migration to use
destroy_all instead of delete_all so the callback is invoked.