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