Change delete_item to RESTful URL.

Notes:

1) Because data-id is used for a number of things, I just added data-update_url at this point. When all of the item.py refactoring is done, I will try to remove data-id (which is the old-style id).

2) diff-cover is showing missing unit test coverage for some of the export refactoring. However, when I set a breakpoint in the code (in particular, in xml_exporter, though it is also true for at least 2 of the of the other "missing" lines below xml_exporter), I do hit that section of code with our existing test_contentstore tests. In fact, test_contentstore is what pointed out that these changes needed to be made. I have e-mailed Will about what appears to be a bug in diff-cover. UPDATE: it's because running cms/lms tests does not count towards coverage of code in common.

3)  I included some of Don's changes to allow has_access to be called on something that is not the course itself.

@singingwolfboy @dmitchell  Please review.