Shared regforms control

[webknjaz]

Moved from GDG-Ukraine/gdg.org.ua#20 / suggested by @zasadnyy

We need to be able to share events among several gdg organizers (or better gdg chapters). Good example is registration for devfest, when at least 2 groups are involved in registration process.

P.S. Actually there's an option godmode which allows users to access all available resources. I suggest promote these users with a right to add users (within ACLs) to some of events.