Add regulation to artefact formats

* Adds new publication subtype from Whitehall
Merge pull request #224 from alphagov/add-regulation-publication-subtype

Add regulation to artefact formats

https://www.pivotaltracker.com/story/show/78637270
Release 20.1.0: Add Regulation format to Artefact
More detail on addition of "regulation" format.
Merge pull request #225 from alphagov/version-20.1.0

Release 20.1.0: Add Regulation format to Artefact
Add a format of `email_signup` for finders

Each finder will have a page responsible for email signups for new
things on that finder.