Allow mentors to leave/review comments for students

[karbassi]

This one needs to be thought out some more, but the basic idea is:

• [ ] Allow mentors to read and leave comments about a student
  • [ ] Only allow mentors to read and leave comments about students in that specific class. For example, if a mentor is signed up for class number 123, the mentor can only read/write comments for the students enrolled in class no. 123.
  • [ ] Only allow mentors who have passed the background check completed
• [ ] The comments are write-only for mentors. Mentors can write a comment, but they cannot edit it.
• [ ] Comments are tied to the mentor and timestamped.
• [ ] Comments need to be approved by an admin. They are visible to that mentor-only until approved by a mentor, which makes the comment visible to all mentors (of that session)
• [ ] Allow admins to 'delete' a comment (is_active). Don't actually delete, just hide from all users. We'd like to keep a record of bad actors, if something happens.

Model (WIP)

class StudentComment(models.Model):

    # Relationships
    mentor = models.ForeignKey(MENTOR)
    student = models.ForeignKey(STUDENT)
    session = models.ForeignKey(SESSION)
   
    # Fields
    body = models.TextField(max_length=1000)
    is_active = models.BooleanField(default=False)
    is_approved = models.NullBooleanField(default=False)
    created = models.DateTimeField(auto_now_add=True, editable=False)