Form url in editorial_remarks/new is now explicit

Because of <%= rails_black_magic %> the form_for helper in
admin/editorial_remarks/new.html.erb was trying to use a route
name which didn't exist when used in the context of a document
collection.
Merge pull request #999 from alphagov/fix-editorial-remarks-on-collections

Fixes 500 error on document collection editorial remarks form.

https://www.pivotaltracker.com/story/show/59399628