Make document series an edition

Document series have also been renamed to document collection.

The admin interface and presentation of document collections should work as before.

There are a few remaining tasks:

* [ ] When a published document is added to a collection, then the search index for that document should be updated.
* [ ] redirect .../.../series/... to /.../collection/...
* [ ] consider whether we need a document collection index page? (currently redirecting to publication index)
* [ ] verify importer behaviour
* [ ] copy the translation of the word 'series' to the translation of the word 'collection'
* [ ] does any cleanup need to happen when deleting a document series (esp of groups)
* [ ] should you be able to delete/unpublish a document series that has documents in it already? (previously you couldn't)
 
https://www.pivotaltracker.com/story/show/52418761