Feature any published edition on world location pages

This extends the new featuring UI to support searching for and featuring
editions that are outside the ones explicitly associated with your world
location.

(This will prevent weird cases where the article covering Margaret
Thatcher's death was linked to every world location that wanted to
feature it.)

This is the first half of the change on the ticket: the second half
is to do with moving the organisations to use the new featuring code.
It's worth splitting this ticket up into two pull requests as the
functionality is distinct.

As well as reviewing the code, I'd appreciate someone checking through
the admin UI for selecting editions and suggesting any improvements they
think of.