58867026 govuk styling

This change is for:

* https://www.pivotaltracker.com/story/show/58810098
* https://www.pivotaltracker.com/story/show/58867026
* https://www.pivotaltracker.com/story/show/58867014

Splits admin interface into two namespaces (one for notes, one for synonyms) because they use similar structure, but the views are different.

Please merge with backend change, as we needed some additional information exposed via API to support new designs (alphagov/trade-tariff-backend#118)