Index the predicate of relevant_to_local_government not the attribute

The attribute reader is edition_instance.relevant_to_local_government, the predicate is edition_instance.relevant_to_local_government?.  All editions have the attribute reader (and AR gives us the predicate for free), but for edition types that CanApplyToLocalGovernmentThroughRelatedPolicies we overwrite the predicate method to calculate the value based on the related policies, we *don't* overwrite the attribute reader.  When indexing we have to use the predicate as there's no guarantee that the value of the attribute and calculated predicate will match up.

There are other approaches:

1. overwrite the attribute reader too - it's not clear what drawbacks this might have
2. make sure the attribute has the same value as the calculated form (via after save callbacks on the policies) - this might get complicated quickly
Construct relevant_to_local_government editions correctly in factories

For edition types that CanApplyToLocalGovernmentThroughRelatedPolicies when we ask to get an instance from the factory that is relevant_to_local_government: true, it's not enough to set the attribute, we want to construct a policy that is relevant_to_local: true and then related it to the edition we are constructing.
Take local government relevancy from *published* related policies

It was just latest_edition before, which would mean a draft policy that changes the relevant_to_local_government flag would change the value of a published edition related to it, and that seems wrong.
Provide a scope on Edition to find relevant_to_local_government content

This knows how to find editions that just use their normal db attribute and those that include Edition::CanApplyToLocalGovernmentThroughRelatedPolicies and get their flag from their related editions.
Update document_filter/mysql to have parity with rummager impl

The mysql filter would exclude local government results if not told otherwise, the rummager impl includes them if not told otherwise.  Rummager is correct, so we have to update mysql.
Prove that the policies controller deals with local gov flag
Merge pull request #487 from alphagov/relevant_to_local_government_fixes

Relevant to local government fixes