Change save_item to be RESTful.

Part of STUD-847.

@dmitchell (and @singingwolfboy if interested), this is ready for review.

Notes:

1) I  have created https://edx-wiki.atlassian.net/browse/STUD-929 for the fact that the payload being sent back on save is oddly structured.

2) I again left IDs alone (except for the section model, which definitely didn't need them). I will next be tackling create_item, and after that I will go through and delete as many old-style IDs as possible.