Distinguish between pending signatures and invalid ones

[tttp]

Hi,
Right now, if you put an invalid email and sign, you will be on hold as a contact, but your signature will still be pending.

To be able to know how many contacts we need to reach to get them to optin (or optout) we need to update the status to "Cancel".
It doesn't have to be done realtime, but have a sql/cronjob (you can use sqlreport for that?) that automatically update the activity status for pending signatures -> Cancel where email on_hold

[tttp]

@rthouvenin I assigned to you for triage, and because I'm not sure we need to add that as one of the stat data.

Might be good to store it?