Explore allowing organizers to refund donations/invoices

I think this feature is almost robust enough for users. And it'd be a handy feature for them.

The issue proposes allowing organizers to refund donations or invoices but does not identify files, tests, or an entry point. First clarify the expected refund behavior and scope; the work is done when the supported refund flow is defined and verified for both donations and invoices.

Written by the indexing model from the issue text.