Let the geonode admin to configure data and metadata download formats

Add configurations to let the GeoNode admin to select which data and metadata formats are available for download.

The following settings are added (documentation is included):

DOWNLOAD_FORMATS_METADATA = [
    'Atom', 'DIF', 'Dublin Core', 'ebRIM', 'FGDC', 'TC211',
]
DOWNLOAD_FORMATS_VECTOR = [
    'JPEG', 'PDF', 'PNG', 'Zipped Shapefile', 'GML 2.0', 'GML 3.1.1', 'CSV', 
    'Excel', 'GeoJSON', 'KML', 'View in Google Earth', 'Tiles',
]
DOWNLOAD_FORMATS_RASTER = [
    'JPEG', 'PDF', 'PNG' 'Tiles',
]