Add Feature#end! method to end a feature
Fixup Feature <-> Document association

Editions have never directly been associated with Features
Unpublishing an edition also unfeatures it.

Prior to this, unpublished documents were still being featured on
organisations and world locations. This was causing 500s on front-end.
Merge pull request #1044 from alphagov/50033335-unpublishing-removes-featurings

Unpublishing removes featurings