Make document series an edition

https://www.pivotaltracker.com/story/show/52418761

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:

* [x] Merge in master .. @elliotcm can you help? 
* [x] redirect .../.../series/... to /.../collection/...
* [x] consider whether we need a document collection index page? (currently redirecting to publication index)
* [x] only display membership of published document collections (membership of a draft collection should not be displayed in a documents metadata)
* [x] fix importer behaviour as described below
* [x] copy the translation of the word 'series' to the translation of the word 'collection'
* [x] does any cleanup need to happen when deleting a document series (esp of groups)
* [x] should you be able to delete/unpublish a document series that has documents in it already? (previously you couldn't)
* [x] rename documentSeriesDocFinder in document_collection.js
* [x] various fixes detailed below
* [ ] check filters work properly after the fixes go in from other tickets

## Importer behaviour fixes

When importing a document which is tagged to a document collection, there are three possible scenarios:

- the collection does not exist at all. This is an error and the import row should be rejected
- the collection does exist and the latest edition is a draft. In this case we add the document to the appropriate group in the draft collection.
- the collection does exist and the latest edition is published. In this case, we create a new draft from the published edition and add the document to the appropriate group in the draft collection.

Note that this workflow means that the document collections will need to be manually reviewed/published before the imported documents will appear in the collections.

## Search fixes

When a published document is added to a collection, then the search index for that document should be updated, this is split out to a separate ticket:

https://www.pivotaltracker.com/story/show/58228020