Ordering publication attachments

Tracker: https://www.pivotaltracker.com/story/show/51889627

Publication attachments can now be ordered. Ordering is restricted to publication attachments only at present, which means we don't have to worry about the fact that the inline attachment markdown tags are tied to the order of the attachments. If there was more time left in the sprint, I would have had a crack at reworking the markdown tags so the ids weren't based on the order, but getting this delivered by Tuesday is higher priority.