Ensure that email addresses of users are normalised to unicode

[tchoutri]

Chrome sends IDNs converted to punycode, but we don't normalise those in the backend, which means that the database may be able to accept two user accounts with the same email address, once in punycode and once in unicode.

Since this conversion step is losseless it's not a problem. Email gateways however will (a lot of times) prefer punycode.