Add new user type (TA), allow them to escalate comments, allow instructors to resolve "reply requests"

[JumanaFM]

TAs have limited accessibility compared to instructors. They have a special identity (icon) to distinguish them from students. They can handle comments by replying to them or escalate comments to instructors attention by clicking on "reply request". Instructors can get a filtered list of "reply requests" that show comments escalated by the TAs.

Reply requests will be marked as resolved once the instructor handle them.

[JumanaFM]

For instructions https://github.com/haystack/nb/pull/272#issue-1580422024