Invite nongov users
At the moment, the only way for us to allow users without a *.gov.uk email address to use Notify is to whitelist their entire domain potentially disallowing access for gov users.
This changes makes it possible to *invite* users from any domain into a service. However, they still cannot register without an invite if they are not on the whitelist.
Invited non-gov users can not change their email address. However, I am not sure that they should be able to. Perhaps in users-profile they should not see edit button.