Fixes 500 error on document collection editorial remarks form.

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