Add instructor/student property for users on backend

[jwjulie]

Create new backend property users tagging them as instructor or student. Change files in src/groups/ and/or install.js to implement Instructor and Student groups to categorize Q&A forum users.

Effort: medium
Downstream Dependencies:

• Role badge: Instructor users have a badge by their name.
• Anonymous posts: Instructor users can see all post authors while students cannot.
• Post resolution: Threads with an instructor response are marked as resolved.
  Acceptance Criteria: Student and teaching staff users/posts can be differentiated at a glance.