Add default unresolved status to threads started by students

[jwjulie]

Change src/topics/create.js, which handles creation and posting of topics (forum threads), to automatically tag a topic with unresolved when posted by a user in the Students group. Add a functino to src/users/index.js, which handles general exported functions from modules in the users directory related to user information, metrics, actions, and permissions, to check if a user is in the Students group.

Effort: Medium, a week
Dependencies: Overlaps with #2 when identifying users as students or staff.
Acceptance criteria: Threads started by students are tagged as unresolved by default. Threads started by staff do not have a unresolved tag.