Add fees_owed and fees_paid fields to School.

This adds `fees_owed` and `fees_paid` fields to the School model. It removes the other payment related fields. It also adds a custom serializer field that serializes Python's `Decimal` object into a plain float.

Haven't dealt with automatically adding the fees on School creation; will leave that for a followup PR.

**Test Plan**: Added unit tests to verify the field was present, and that it was read-only.