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
You have valid point about not showing these warnings for published documents. I'll correct that.

Why do you think the query will return false for the second document? `this-is-a-slug--2` is `LIKE` `this-is-a-slug--%`. I'll update to let ActiveRecord do the interpolation.