Store user email addresses into server db

We want this for two reasons:

1) In the migration from v1 to v2 it would have been useful to have user email address to let people know of the change. 

2) With Apple review of Neebla, I'd like to know which Apple emails have been used in review. So I can tell Apple which emails have been used-- so they can just sign in and not have a failure redeeming an invitation.