Give NavAppPlugin its own HTML

In CMS-13486 the NaAppPlugin was added.

We need a NavAppPlugin as component because every frontend plugin application must be a Wicket component. At the moment this component does not have it's own HTML  By adding it I think we can get rid of a lot of the complexity that we currently have with the HTML resources. The navapp resources are conditionally added by using a FilteringHeaderResponse (see Main#addHeaderResponseDecorator()). I think we can completely get rid of the decorator.