This prevents people re-submitting the form multiple times. Although the
backend validates it, we show an ugly alert message that is hard for
users to understand.
During registration, use the given email address only as public visible
profile email address. Set the internal notification email address
`enterprise.email` to the owner's email address. That can be changed
later.
Sets the public email address to the given email address as well as the
internal enterprise.email (notification email address).
The field is not mandatory any more since enterprise.email is populated
from the owner's email address if no value was given.
