WebUI: check principals in lowercase
    
    WebUI checks whether principal name of logged user and principal name
    in each command is equal. As KDC for our principals is case insensitive
    - it does make sense to switch this check also into case insensitive.
    So both principals are reformated to lower case and then
    compared.
    
    Part of: https://pagure.io/freeipa/issue/3242
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>