Remapped edition display_type for feed titles

I've gone for a simple solution this time, as it's a localised change
for feeds. We can expand it into something a little more standard and
make it part of the domain as the requirements grow.
Merge pull request #392 from alphagov/remap-display-types-for-feed-titles

Remapped edition display_type for feed titles

https://www.pivotaltracker.com/story/show/46700561