Bump govuk_content_models to 16.1.0

https://www.pivotaltracker.com/story/show/67644106

This bumps govuk_content_models to 16.1.0, which includes changes to tags so that they are created as `draft` by default. 

To fix all the tests which break as a result, I've updated all the tests which use the `:tag` factory to instead use the new `:live_tag` factory.