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)