[DISCUSS] Redesign admin edition show page

[Note – we are planning to show this to editors before merging into master, but I have put it here to start getting feedback.]

https://www.pivotaltracker.com/story/show/53973095

The main change is that you can no longer preview documents and document metadata on the admin edition page – you have to go to the preview on the frontend.

This includes the start of some cleanup of the admin CSS. The huge global CSS files can be migrated to helpers or views, similar to frontend CSS.

Known issues which should be separate stories:

- Supporting pages need a cleanup, similar to how translations have been done
- Some buttons have a `data-confirm` attribute on them to show a confirm message, but this does not seem to work.