Set autosave on has_many to avoid validation issue

When we save the association we can run into a condition where the
image is missing through virus check, or on preview and development
environments where we don't copy the assets.

Unless we use an `autosave: true` on the has_many association, we run
into a situation where we cannot save the join model correctly and we
end up missing associations when republishing.

This can also be fixed by sorting out the image validations on the
`edition_world_location` model, but this is a cleaner solution.

Also moved the association off the main edition model and into the
module for neatness.

Ticket: https://www.pivotaltracker.com/story/show/45879539