WebUI: add default on_cancel function in adder_dialog
    
    Adder dialog is mixed with confirmation_mixin. That mixin calls on_cancel method
    when closing dialog using ESC key. Previously the on_cancel method
    was not defined, therefore dialog was not correctly closed. This was the root
    cause of the bug, that adder dialog cannot be opened after closing it using ESC.
    
    Now the default function for on_cancel is dialog.close. So dialog
    is correctly closed.
    
    https://fedorahosted.org/freeipa/ticket/6388
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>