Whitelist domains / domain admins

[michaelsnook]

It would be great if the organizations and then organizational admins could manage the process of adding new people into Lumen so that our organization didn't have to manage that all centrally. For example, if everyone at activist-org.net is welcome into the discussion platform, we would whitelist activist-org.net and then folks from that org could create accounts or invite new staffers from that domain.

(This might also mean we'd want to be able to lock in an email address so that people can't change it... but I think that will be a separate issue addressing separate concerns.)