Ticket 48926 - fix "expiresin" entry definition
    
    Bug Description:  The "expiresin" component incorrectly set the model to "0",
                      which causes unexpected behaviors in the account policy tab.
    
    Fix Description:  Set the model to "null" to trigger the correct behavior for this
                      component type.
    
    https://fedorahosted.org/389/ticket/48926
    
    Reviewed by: nhosoi(Thanks!)