NHT: Send an email to the uber-admin when a new User is created.

We should _at least_ notify the first user (the "main" admin) when new users are created/approved.

Maybe also notify the other admins? @justinbmeyer is there a way to differentiate between "uber" admins, and normal ones? Or would that just be by UID?

Any reason to not notify all admins? I assume once https://github.com/donejs/bitballs/issues/345 is done, some users that were previously admins no longer need to be admins, so notifying all admins is probably acceptable.