Adding an IsInstructor Function into src/privileges/users.js

[emshyu]

The main components of this pull request are an IsInstructor function to check if a user is considered an instructor and test cases for it in test/user.js. This is intended for the highlighting/flagging instructor posts feature.

However, although the frontend (it has visible design changes) and backend are working separately, we are still running into issues with calling the function in posts.tpl and topics_list.tpl. We also went to office hours for TA help and were unable to call the function successfully in the frontend.

Context:
Before choosing this, I experimented with various options of tracking if a user is an instructor including adding an isInstructor attribute to user in the database, adding fields to postData, and making functions in src/posts/data.js and src/posts/user.js.

I eventually decided to try making a function in user privileges because I saw that frontend template files could successfully call privileges.isAdminOrMod, which checks if the signed-in user is an administrator or moderator. Hopefully, with more experimentation, we can call the function successfully.