Tweak audit trail methods to allow eager loading (and use it)

This took loading the audit trail for an edition with ~700 versions down from 2628.6ms to 552.8ms.  The next stage for pagination is to make #document_audit_trail more fully SQLized (right now it does a lot of ruby in-memory stuff).
Split history & notes tab on admin into 2 separate tabs

Might need to think this through as the editorial remarks are across all editions of the document and maybe they are less useful without the context of history around them.
Paginate the version audit trail for editions in admin
Make audit trail pagination come use ajax to avoid reloading the page
Split editorial remarks

Show those for this edition at the top and those on other editions underneath.  Heading is "on previous editions" which might be a bit of a misnomer when viewing old editions.
Add simple feature to describe audit trail on editions in admin
Merge pull request #567 from alphagov/audit-trail-on-admin-improvements

Audit trail on admin improvements