MDL-43267 JavaScript: Improve accessibility of course editing and drag/drop

This commit brings the course editing interface more in line with ARIA
guidelines.

This adds aria-describedby tags to many of the actions such that they
describe what they're editing.

It also removes the aria-grabbed attribute as we do not provide the full
aria drag/drop descriptions; instead we provide an alternative interface to
supplement the mouse drag/drop.

This change also applies to block drag/drop.