Adding migration to change making-schools-and-colleges-more-accountable slug

The migration changes the slug of the policy document, removes the old documents from search, adds the new paths to search and sets a redirect on the main policy slug.
Update policy data migration to use prefix route

Use a prefix redirect for the policy change. This will then match all
the supporting pages and redirect them.

Also remove the 'for' from the search index calls. As far as I can tell
the search index gets set based on the object anyway. It was also
causing the migration to fail for me locally.
Merge pull request #1904 from alphagov/change_schools_colleges_slug

Change schools colleges policy slug