GEOS-5844 fix nulls in SecureCatalog.list

a catalog filter can potentially result in nulls being in the
returned iterator. an example is a LocalWorkspaceCatalogFilter
when a WMS virtual service getcaps request occurs.

forward port from 5edc2cc67de9aa922c798c1e8049939178a9d529

had to add means to override check on GeoServerSecurityManager
so test cases would not depend on full spring context
GEOS-5849 added localized rule labels support to GetLegendGraphic output
GEOS-5847 fixed issue with nested layergroups
Ensure network links are always getting downloaded, without shrinking the bbox
GEOS-5851 Make sure we don't synch up computing a non cacheable feature type each time we need a feature source
Fix build
GEOS-5852 default filter chain does not create HTTP session even with session creation enabled
Security documentation enhancments
trying fixing the build after GEOS-5847 broke it
GEOS-5862 - GSUSER variable not available in REST requests
GEOS-5863 Add support for white space margin in graphic fills
Updated test axis order and added srsName check for [GEOT-4471] App-schema: filtering by xlink:href causes ClassCastException if the type has native CRS
[GEOS-5865] Make GeoServer leverage the GeoTools multicoverage raster API
Avoid dispose warnings when feature type infos are getting disposed with the new keys taking into account projection policy
[GEOS-5866] KML feature downloads may fail when GE asks for them looking at the entire world
Fix style generation, cache JAXB context as it's expensive to create
Unplug sequence generators that are probably not neededd anymore
Comment out generators that we should not really need
Make sure the request bbox is respected
Allow to control the label size from SLD