New featuring code on organisations

This allows organisations to use the new featuring code. The old
features will be migrated across to use the new feature data model.
Organisations will now be able to feature anything they like on the
site, without having to have an explicit association.

Note: as mentioned on the ticket, the Wales Office (and any other
organisations which use multiple translations) will now have separate
featuring for their two homepages (Welsh, and English). I've duplicated
the current set, but when they next feature something they will need to
feature it on *both* the "Featured Documents" and "Featured documents
(Cymraeg)" tabs.

https://www.pivotaltracker.com/story/show/48727749