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
This also makes me wonder if we need to rethink the query a little. It wouldn't be useful to have the warning appear on the original document. It should only appear on a new document. It also makes no sense for the warning to appear on a document that has already been published because you have no control over the slug once it has been published - changing the title will not change the slug.