Tweak promotional feature item edit page title
Don't make promo item title a link if not present
Display the existing image when editing items

I'm using the `image_url` method to check for the presence of an image
as `image.present?` doesn't play nice with the way files are stored
and virus checked in production environments.
Promo feature can be created with first item

This is almost nested attribute inception, but the workflow makes more
sense when creating a promotional feature
External links are marked as such
Tighten up url validation and correct typo
Merge pull request #453 from alphagov/promotional-features

Promotional features

https://www.pivotaltracker.com/story/show/46261745