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
Here's a failing test:

```ruby
  test "#has_identical_slug? warns of slug clashes" do
    news = create(:news_article, title: "Big news")
    duplicate = create(:news_article, title: "Big news")

    assert duplicate.document.has_identical_slug?
  end
```