See that a post has been edited

Adds three database fields to track in what revision and by what user a post's
content was most recently edited, and some template code to display that to
a user. Adds restriction so only the post creator or an admin can edit a post.
Currently stores but does not display who last edited the title, it will fit
in the interface on during UI refactor.

Change-Id: I92577798d4f9da7fa916cd1fede93e6c171c7ee3
Merge "See that a post has been edited"