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
Can we rename this to `has_similar_slug?` It's not technically identical.