Automatically refund when a registration is cancelled/rejected

[dunkOnIT]

This would make organizer's lives easier, while also introducing some risks because it deals with automatic actions taken on payments. Thoughts:

1. We may want to add a popup with a tickbox that says "Also refund registration" - a good in-between option that let's organizers not always have the refund be processed [this could also act as an "Are you sure you want to cancel/reject?']
2. If we passed in a refund: true argument in the update payload we could have a hook as part of the update process that triggers a refund. This would make it quite easy to allow an organizer to trigger multiple refunds at once - but whether it is easy is not as important a question as whether it is secure