Email all volunteers

Issue Summary

I was thinking it would be useful to have an "Email all" link next to the roster that takes the event coordinator to a form where they can send an email message to all of the volunteers who are signed up for the event. I guess it would be similar to the email form in organic groups. That way, the coordinator could quickly send an email to the volunteers a couple days before the event to, for example, remind them what to bring or to ask if anyone has extra room in their car if someone wants to carpool.