Add a "source" field and possibly a "retracted" field to nomcom.volunteer

[rjsparks]

trafficstars

It would be useful for nomcom chairs to be able to see that a volunteer volunteered by checking a box in the registration form at a particular meeting or directly through the datatracker.

It might also be useful to retract volunteers (a "retracted" timestamp field) rather than delete them.

cc @larseggert, @richsalz

[richsalz]

It could also prevent questions "I didn't clock the box" "Yes, you did two IETFs ago" "Oh, ok" :)