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

fixes #12595
ref #12620 

cc @blizzz @DeepDiver1975 @PVince81 @LukasReschke 

re-incarnation of #12850
what does display name changing have to do with this?