Sortable has-many forms: Revive PR #881

This is a feature, I'd love to see get brought in, but #881 hasn't seen any activity in months. I've rebased the effective changes with the current master as a starting point on fleshing out this feature. I haven't tested this in any real projects yet, but might tackle it this morning. If anyone has any thoughts on implementation/styling/whatever, feel free to let me know.