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.
WIP on search index updates

As discussed with @bradleywright this is an in-progress change on search index updates based on whether a policy is relevant to local government or not. The tests currently aren't working, and someone needs to pick this up and complete it.