Allow admin to change personal price

[pdpinch]

As a reviewer, I need to change the personal price for an applicant.

Sometimes I cut the price, to apply a scholarship. Sometimes I raise the price, when the applicant has agreed to an add-on purchase (e.g. access to extra special videos)

There is already a model for personal prices that works, but there's no UI for it outside of the django admin.

Design

TBD

acceptance criteria

• access should be limited to a subset of reviewers.