Otherwise, it is possible that a `User` primary email is successfully
saved, but it cannot be added to the emails table due to being invalid
for the `Email` model.

The `Email#email` validation is now more relaxed, and the same as the
`User#email` validation: it is the Devise email validation, that
only checks that there is a single @ sign separating local
part and domain. The previous stricter Email#email validation was
enforcing compliance with RFC3696, but many mail servers allow addresses
that are not fully compliant with the RFC. Therefore, the relaxed
validation is preferable, to avoid blocking working email addresses.

Changelog: fixed