Adds Regulation publication subtype

Bumps govuk_content_models gem to version 20.1.0 so that Panopticon can handle the new publication subtype.

In support of story https://www.pivotaltracker.com/story/show/78637270
A govspeak major version bump could be a thing.  Looks like it was introduced by @kalleth here: https://github.com/alphagov/govspeak/pull/39/files

Worth speaking with him to see what the impact might be.