Edit Event Fee

Allow participants to edit a pending event registration.

It re-uses the backend Edit Participant screen, with a few simplifications (forces sending a receipt, for example), and also does a few access control checks to allow editing using a contact checksum.

It also allows partial payments, on a separate form, if they have a link with a checksum (the link is displayed on the "View Contribution" admin page). It is used in another circumstance where someone registers multiple participants to an event, but each participant will pay their own fee. It can also be useful to require that a deposit be paid before a certain date.