Warn about identical slugs

www.agileplannerapp.com/boards/173808/cards/5403

As a department editor
When creating a document in Whitehall Publisher
I want be warned of possible title duplicates
So that I can create unique titles and avoid ugly --2, --3 slugs

This warning appears in 2 places:
- on the edit form
- on the edition show page, below the submit / publish action buttons
Up to now, the translation files have only serviced the frontend. I'm reluctant to introduce the pattern of storing static, non-translated content for amin pages in the translation files for a number of reasons:

1) We share the translation files with FCO to get translations for the public-facing pages. If we start to fill the translation files with admin-related content, these will get translated unnecessarily, creating unnecessary work. Unless we tell FCO specifically not to translate sub sets of keys, which is more work and more potential for user error, inconsistencies.

2) It will make it harder to split out a separate user-facing application when we start to break whitehall apart.

3) The YAML files are a pain to manage and work with/regenerate, etc.

4) It introduces another pattern for developers to have to think about. What do I call the keys? Where do I store them? What content should go in there and what shouldn't?

5) It adds a level of indirection when working with the templates, and some may find this confusing, specifically front-end developers that may not be familiar with how Rails translations work.