Change save_item and create_item to be RESTful.

Things to test:

1) Create, delete, modify, and re-order static pages.
2) Create, delete, and re-order sections, subsections, and units on the overview page. Test changing release date and grading type for subsections.
3) Create, delete, and re-order units on the subsection page. Test changing release dates, grading type, and name of subsection.
4) Create, delete, modify, and re-order components on the unit page.
5) Test public/private and drafts on the unit page.
6) Test creation and deletion Course Infos on Course Updates.
7) Test modification of handouts on Course Updates page.

@dmitchell @singingwolfboy Please review.