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 query isn't quite doing what we want - if you have an existing document with the slug "this-is-a-slug" and a new document is created with the same title, the second document ends up with the slug "this-is-a-slug--2". Calling this method on the second document results in the following query:

```"SELECT COUNT(*) FROM `documents` WHERE `documents`.`document_type` = 'Policy' AND (slug = 'increasing-opportunities-for-young-people-and-helping-them-to-achieve-their-potential' OR slug LIKE 'increasing-opportunities-for-young-people-and-helping-them-to-achieve-their-potential--%')"```

Which will return false. It will return true for the previous document though, which isn't where we want it.

Also, you should let ActiveRecord do the parameter interpolation for safety sake, e.g:
```ruby
scope.where("slug = ? OR slug LIKE ?'", slug, (slug_without_sequence+sequence_separator)).count > 1
```

Less of an issue here where the parameters aren't user input, but good practice all the same