Fix previews

I'm not sure, but this *might* work.

I had to upgrade the version of gov_uk_content models to pull in the latest version of our content models, but this bought in a bunch of other dependencies, which I *think* has broken authentication. 

Looking at https://github.com/alphagov/govuk_content_api/pull/156 and https://github.com/alphagov/govuk_content_api/pull/157, this seems to have been a breaking change for the GDS lot, so I've added those code changes and bought some of the dependencies into line with everything else.

I can't be 100% certain this works, as I can't get it working in dev, but I can't see this making things any worse, so I figured why not give it a try?