Rename document series to document collection

Including a database migration to rename the tables.
Use correct url helpers for document collections

Document collections should now use public_document_path which
calculates the url from the slug of the document.
More work to make tests pass doc series->collection rename
A few more test fixes
Tidy up document collection test
Remove warning about changes going live

They no longer go live immediately.
Fix up cucumber tests
Simplify document collection presentation

The presenter classes only existed to inject presented versions of
Editions. This was a lot of mechanics for something quite simple. It's
simpler to do this with a few lines in the controller.
Fix failing JS test

Fixes pluralisation failure in renaming series->collections
Allow DocumentCollection to be used in admin filters
Set meta description on doc collection show