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
There shouldn't be an impact to Panopticon – legislative lists are only in use in Whitehall and Specialist Publisher, neither of which export unrendered govspeak from their own systems.