MaxFeaturesForPreview added to WFSAdminPage and MapPreviewPage

Continuation of https://github.com/geoserver/geoserver/pull/790
Added the ability to control the default maxFeatures parameter on WFS requests made through the Layer Preview page. This parameter is controlled from the WFS Admin page and the default value of 50 is preserved.
Changes:
Added Max Features for Preview parameter to WFS Admin Page
Modified Map Preview Page to use this parameter when running WFS requests
Added test cases for new features.