WIP: Add document show page to admin

This is a work in progress from the design work in [Pivotal ticket #62145592](https://www.pivotaltracker.com/story/show/62145592).

The basic premise is that edition pages should instead be document pages: they should give you an overview of all of the editions in a document to make the lifecycle of a document clearer to understand.

 - [x] Add basic document show page
 - [x] Creating new edition should redirect back to document page
 - [x] Add link to get back to document page from edition edit page
 - [x] Add publish and second eyes buttons which redirect back to document page
 - [ ] Move change note field from edition edit page to document page, as part of second eyes/force publish process
 - [ ] Move import process to a separate controller or make it work on this page (`edition.imported?` state in current edition show page, speed tagging etc)
 - [ ] Second eyes: submitted/rejected states should display correctly
 - [ ] Errors: they should should be displayed somewhere, perhaps with some thought about how to make them a bit less of a mess than they are at the moment
 - [ ] Scheduling: it would be neat if there was a "scheduled" state and scheduling controls were on the document page
 - [ ] Editorial remarks: needs ajax and stuff.
 - [ ] Translations: they should probably be moved to the edition page
 - [ ] Supporting pages: they should probably be moved to the edition page
 - [ ] Translation previews
 - [ ] Some thought should be put into what audit trail entries should be displayed to make it make sense on this page
 - [ ] Fact checking