Return to alphagov govuk_content_models

First part of upgrade all the things, let's get back to using the main alphagov content_models.

Need to do this incrementally else we'll be changing too many things at one time, so this PR get's us to govuk_content_models 6.0.6. I had to upgrade some mongo things as well so I would advise we merge this in the morning @pikesley so we can reboot anything that needs it.