Relevant to local government fixes

After reviewing #483 and refreshing my memory about how the relevant_to_local_government toggle works on the search pages I realised 3 things:

1. we're still using the mysql based document filter (not just in development, but we use it on production for non-english content index pages)
2. it didn't have parity with the rummager implementation for coping with that flag
3. we weren't indexing the relevant_to_local_government flag properly if it was calculated from related policies

This bag of commits fixes all that.

No pivotal ticket exists for this work.