Adds support for a secondary 'proxy' OGC_SERVER config in settings.py to allow a second 'public' URL to be used for public layer links.  Add support for write-only GeoServer.

Example configuration below.  Code is changed in appropriate locations (by my best guess) for an OGC_SERVER config (such as below), where 'PUBLIC_PROXY_ENDPOINT_ENABLED' == True, and use the LOCATION value from there instead of 'default'.   

Also adds support for write-only GeoServer via the 'CATALOG_WRITE_ENABLED' flag.  If false, lines in layers/models.py are commented out to prevent GeoNode from updating any information in the backend GeoServer catalog.    

  'proxy' : {
        'BACKEND' : 'geonode.geoserver',
        'LOCATION' : 'http://localhost:8080/geoserver/',
        'USER' : 'admin',
        'PASSWORD' : 'geoserver',
        'OPTIONS' : {
            'MAPFISH_PRINT_ENABLED' : True,
            'PRINTNG_ENABLED' : True,
            'GEONODE_SECURITY_ENABLED' : True,
            'GEOGIT_ENABLED' : False,
            'WMST_ENABLED' : False,
            'PUBLIC_PROXY_ENDPOINT_ENABLED' : True,
            'CATALOG_WRITE_ENABLED': False
        }

This has been tested on our own GeoServer instance, however an additional check of which links should be proxied from GeoServer would be useful.