[Discussions] Sort threads by date of initial post

Problem statement:

In the "legacy" discussions format, each individual post showed in it's own box, and the posts in a thread displayed with boxes connected. In the "new" format, there's nothing to visually separate posts or connect posts in a thread. This makes it difficult for some users to follow discussion threads; showing threaded responses only by indent is an accessibility problem.

Proposed solution:

The "new" discussions needs a way to do both of the following: (1) show some kind of divider between individual posts in a thread and (2) show a connection between threaded posts. This would provide the visual cues some users rely on to follow threaded discussions.

User role(s):

instructor,student