Revert "Unify the email validation of UserUtil::isValidEmail() and Mailbox::__construct()"
Revert "Unify the email validation of UserUtil::isValidEmail() and Mailbox::__construct()"

This change broke WCFSetup, because Mailboxes implicitly carry a language which
is not readily available in WCFSetup.

The validation of the email address should likely be moved into a dedicated
method in Mailbox and then UserUtil::isValidEmail() deprecated.

This reverts commit 395f9a6affffdd5381134d3f9d39b70de453b668.

SoftCreatR/WCF,SoftCreatR/WCF,SoftCreatR/WCF,WoltLab/WCF,WoltLab/WCF,WoltLab/WCF,SoftCreatR/WCF,WoltLab/WCF