Handle cases where a VE who has signed is then unassigned from a candidate

If a VE signs and is then unassigned, their signature remains, and if they are added again it is as though nothing had happened: their signature shows on the forms.

This led to awkwardness in a session: nobody knows why all the VEs were unassigned from a candidate, or if it was done on purpose, and there was question as to whether it was appropriate to assign them back again due to their signature showing as soon as they were assigned.

I suggest a comment be required when unassigning someone who has already signed, with an option as to whether to repudiate their signature, and that VEs who have signed but are not assigned be displayed in the signers list as such.