Mentorship program

## PLEASE DON'T MERGE THIS YET
# Mentorship Program

This branch will have all the changes for the mentorship program. This is work in progress. Please provide feedback.

### Tasks
- [x] mentors can read all papers
- [ ] filter papers so mentors can only read the *"mentors can read this"* papers
- [ ] create admin section to map mentors to papers
- [ ] create admin section to mark users as mentors (question [1])
- [ ] send mail to mentors once they're approved as mentors. (question [2])
- [ ] mentors can comment on papers? (question [3])
- [ ] insert your request here

### Questions
1) Currently each ``User`` has a ``mentor`` boolean field, but probably want to add a ``mentor_id`` field to the ``Paper`` model.
In that case, the ``mentor`` field for User doesn't make much sense. Maybe we could create a ``before_save`` hook for ``User`` that updates the field accordingly.
2) are we still doing this?
3) How's paper feedback going to happen? Mentors and speakers should talk to each other as they like or should we add comments on each paper? (only readable by admins and mentors)



### The rules
- All mentors can read all papers that are marked ``mentors_can_read``