Rework association between series and documents

Instead of linking series with editions, we now link directly to the
document. This removes the need to copy across the association with
each new edition and greatly simplifies the code in general. We no
longerr list series documents grouped by type on frontend - this added
little value to the user.
DRY things up: kill off DocumentSeriesFinder

SluggedModelFinder does exactly the same work
Rework Importer to work with new doc series assoc

I'm not entirely happy with the way more code is being chucked into
Import#perform, but without a wholesale refactor of the way the import
engine works, I can't see how else to handle this.
Remove fields for doc series from editon form