Add UK aid logo to DFID page

This is a special for DFID and should not exist for any other
organisation. So I have hard coded it in for just their slug. Added a
cucumber to ensure it only shows for their organisation.

https://www.pivotaltracker.com/story/show/49248309
Merge pull request #640 from alphagov/uk-aid-logo

Add UK aid logo to DFID page

https://www.pivotaltracker.com/story/show/49248309