Add content type to URLs

I think it makes sense for this to be a full-blooded association with
the ContentType model, rather than a string attribute.
Assign content types in UrlsController#show action
Add content type drop-down to URL form
Use select2 for the content type drop-down
Add basic styling to the content type drop-down
Merge pull request #33 from alphagov/content_type_dropdown

Add content type dropdown
Merge branch 'master' of github.com:alphagov/transition into add_user_needs

Conflicts:
	app/assets/stylesheets/frontend/views/_urls.scss
	app/views/urls/show.html.erb
	db/schema.rb