Username check with @ should consider e-mails

User names are extremely filtered. Therefore the registration via certain emails is not possible. The username check must consider e-mails

Especially assiatic emails are not possible at the moment.

• https://dev.quiqqer.com/quiqqer/quiqqer/blob/master/lib/QUI/Users/Manager.php#L345
• https://dev.quiqqer.com/quiqqer/quiqqer/blob/master/lib/QUI/Users/Manager.php#L1633 (checkUsernameSigns)