Bypass auth.email-domains (2)

This vulnerability has the same effect as my previously reported bug [#2224 Bypass `auth.email`-domains](https://hackerone.com/reports/2224), but uses a very different approach, bypassing the current fix. Instead of truncating through length, this vulnerability uses truncation via MySQL’s (weird) behaviour on inserting Unicode characters with code points greater than `0xFFFF` into columns that have a `utf8` charset. MySQL then truncates a string as soon as it reaches such a character. For more info, see [How to support full Unicode in MySQL databases](http://mathiasbynens.be/notes/mysql-utf8mb4) by @mathias. To replicate, register an account with following address: `[email protected]𝌆@allowed-domain.com`. I would suggest to reconsider not verifying email addresses as was mentioned [here](https://secure.phabricator.com/D8308#5).