CiviMail should have a re-subscribe action

Currently, once a contact unsubscribes from a group, it's treated as any other non-subscribed contact, and the only way for them to re-join the group is to go through the subscribe+confirm process.

This is not optimal, especially that we don't allow subscription to non-public groups, so members of such groups cannot re-join them at all.

We should add a resubscribe action based on a hash (that's unique for every resubscribe action) that would allow a contact that unsubscribed from a group to re-join, even if the group is non-public (and cannot be subscribed to).

Because this action is hash based, it should be trusted and not need a confirm step (i.e., if a hash matches, we assume we can trust the action and resubscribe the contact).