Admins should be able to transfer attendee linkages

[EliAndrewC]

Sometimes we link a PanelApplicant to an Attendee in error and would like to undo it later. The typical case is when someone accidentally creates a duplicate registration for a panelist, and then we want to "merge" the two, by assigning all of the duplicate's AssignedPanelist and PanelApplicant entries to the canonical Attendee and then delete the duplicate.

We should have some form somewhere that allows us to transfer all of one attendee's panel-related links to another.