Infer edition's relevancy to local from policies

We include a new module: this module overrides the edition's
attribute `relevant_to_local_government` with a method that checks
`related_policies` for relevancy. As part of this, also move the
checkbox so it only appears on Policy pages.
Re-index all related editions on (un)publish

Currently we do this in a blanket fashion on publish or unpublish - it
might be better to only do this when the policy's value for
`relevant_to_local_government` changes between published versions.
Fix broken tests by stubbing relevance

This is normally inferred through the parent, but really we only care
about the return value of this methid, so stub it out.
Test we ignore the DB value of relevant_to_local_government
Merge pull request #443 from alphagov/infer-relevant-to-local-from-related-policies

Infer edition's relevancy to local from policies

https://www.pivotaltracker.com/story/show/47972463