Adding migration to rename three visas-immigration subsections

There are no whitehall mainstream categories with these parent tags.
Add a minimal JSON view for artefacts.

This view lets the URL arbiter <https://github.com/alphagov/url-arbiter>
seed its initial data set more quickly. Part of this is because the view
can skip tag lookups; part is simply that there is less JSON to
serialise. The response time on my dev VM dropped from 600-3000ms per
page to 50-100ms.

Tested together with the importer, limited to 20,000 of the 100,000 or
so artefacts, the time taken dropped from 6 minutes to 90 seconds.
Refactor to make code reusable
Index the "unimoney" campaign

At the moment, all artefacts of the "campaign" format are excluded from
being indexed in search. 

We want to index the "unimoney" campaign in search, and we can do that 
by including this in the list of exceptional slugs.
Merge pull request #196 from alphagov/minimal-artefacts

Add a minimal JSON view for artefacts
Merge pull request #197 from alphagov/index-unimoney-campaign

Index the "unimoney" campaign
Merge pull request #193 from alphagov/more_slug_changes_for_visas_and_immigration

Adding migration to rename three visas-immigration subsections