Extract Featurable to module
Extract feature list display to partial
Featuring on organisations

This allows organisations to use the new featuring code. The following
commit will handle the migration of the data from the old to the new
model.
Database migration: move org features to new code

Note that for organisations with more than one translation (currently
just the Wales Office) we copy the existing feature set to both their
new feature lists.
whitespace
Don't display spurious 'locale='
Proper tab display for features + promos
Rejig tab display on organisation admin pages

We now have horizontal tabs again, with a 'more' option showing less
commonly used tabs. This works better with the edition selection
navigation that appears on the featured documents tabs.
Don't show label if no featured documents