Calculate fees based on School's pre_save signal.

This recalculates a school's fees every time it's saved, with the formulate `registration_fee + delegation_fee * total_delegates`.

Question for @sdot5: do we want to use the estimated delegate numbers given by the advisors to calculate the delegate fees? I can imagine some advisors being upset about that. We could tell them that this is just based on the estimate, and will change. Or, we could only do registration fee for now.

**Test Plan**: Added a test case, verified the updating behavior in the admin panel.