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
Tweaks for clarity
Merge pull request #1745 from alphagov/warn-about-identical-titles

Warn about similar slugs