Displays the date, time, and username of the most recent save and publish

Conflicts:
	cms/djangoapps/contentstore/views/item.py
Merge pull request #4342 from edx/benmcmorran/display-last-edited

Displays the date, time, and username of the most recent save and publish
Removes unnecessary PUT after DELETE
Discard changes rewrite.

STUD-1860
Code review feedback (to be squashed).