[Discussions] Show discussion edits as "new"

[lshulman]

Have Canvas indicate to us when an old discussion post has been edited (mark it as "new" again) (the way Blackboard does).

Currently Canvas does not alert users when a discussion post (reply) has been edited. If I am grading student posts and a student edits their post to improve it to get a better grade, I should be alerted to such. I may have missed some student edits because I only look for "new" (unread) posts when I go into a discussion.

Canvas should mark the edited post as "unread" (or "new") (even if the I had previously read the post and unmarked the green "new" button). Maybe even make it a different color for edited posts vs. new posts!