810

Get rid of the Modal/lightbox - relating to issue https://github.com/mesosphere/marathon/issues/810

Introducing a PageComponent inherit from the ModelComponent functionality.
The header css still has to be styled.
Let's get rid of this destroy functionality, which was part of the modal component. Let's just let the `state.activeApp` determine whether to show this component or not in Marathon.jsx, we shouldn't need the `state.pageClass` anymore. Again this is left overs from the ModalComponent. In general try to trim down all the modal functionality.