Removed duplicate searchable in DocumentCollection

DocumentCollection had a searchable declaration, duplicating
one inherited from edition anyway. The second version didn't
restrict to only published collections.

As a by-product, slug has been removed from the index.
Merge pull request #977 from alphagov/fix-indexing-of-document-collections

Removed duplicate searchable in DocumentCollection

https://www.pivotaltracker.com/story/show/59311828