Fixed 500.html

Reverted extension block addition in https://github.com/GeoNode/geonode/commit/d169a68320c4843fc81993c00c6a53984c18c868#diff-c8b010e5d1d03fac97aeb6cd83c4c399

Also added docs for developers to warn against usage of template tags in that file.
Merge branch 'master' of github.com:GeoNode/geonode