Change "items" back to "value", type the validator

Problem/Motivation

In AlternativeUserEmailsValidator, PHPStorm reports Parameter's name changed from 'value' to 'items' for the validate() function.

Changing the name to "items" is confusing because in this particular case, it is the "mail" field, which has only one value.

Also, the functions in the validator do not have specified return types.