Mail address of users is now changable in the user management

* introduced new route settings/users/{id}/mailAddress
* kept old responses
* better error messages
* dropped lostpassword.php from settings/ajax
* cleaned up the UserList.add() and hand in user object instead of
  each attribute as another parameter
* check for change permission of mail address
* proper response messages

fixes #12595
ref #12620

cc @blizzz @DeepDiver1975 @PVince81 @LukasReschke

re-incarnation of #12853