Prevent 'mark as paid' for pending registrations

[tomasr8]

When a registration is moderated and there is a fee or paid items, if you first mark a registration as paid and only then approve it it gets into a strange state where at the top it says not paid but at the same time the invoice shows up as paid.
(More context in a SNOW ticket: INC3861152)

Marking as paid should probably be disabled when the registration is still pending.