Allow usernames to contain @-signs as long as it is not at the begininng

This allows to use email addresses as usernames and fixes issue #772

Author: jbtronics

src/Entity/UserSystem/User.php

@@ -256,7 +256,7 @@ class User extends AttachmentContainingDBElement implements UserInterface, HasPe
     protected ?string $password = null;
 
     #[Assert\NotBlank]
-    #[Assert\Regex('/^[\w\.\+\-\$]+$/', message: 'user.invalid_username')]
+    #[Assert\Regex('/^[\w\.\+\-\$]+[\w\.\+\-\$\@]*$/', message: 'user.invalid_username')]
     #[Groups(['user:read'])]
     protected string $name = '';
 

translations/validators.de.xlf

@@ -164,7 +164,7 @@
       </notes>
       <segment state="translated">
         <source>user.invalid_username</source>
-        <target>Der Benutzername darf nur Buchstaben, Zahlen, Unterstriche, Punkte, Plus- oder Minuszeichen enthalten.</target>
+        <target>Der Benutzername darf nur Buchstaben, Zahlen, Unterstriche, Punkte, Plus- oder Minuszeichen enthalten und darf nicht mit einem @ beginnen.</target>
       </segment>
     </unit>
     <unit id="_v.DMg." name="validator.noneofitschild.self">

translations/validators.en.xlf

@@ -164,7 +164,7 @@
       </notes>
       <segment state="translated">
         <source>user.invalid_username</source>
-        <target>The username must contain only letters, numbers, underscores, dots, pluses or minuses!</target>
+        <target>The username must contain only letters, numbers, underscores, dots, pluses or minuses and must not begin with an @!</target>
       </segment>
     </unit>
     <unit id="_v.DMg." name="validator.noneofitschild.self">