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
Removed block in 500 template
Fixed #1256
Stopped downloading a data.zip.

Just copy it from the generated GeoServer artifact.
Do not rely on data.zip, use geoserver's
Raise exception if map download fails, do not fail silently.
Updated changelog for version 2.0c4
partial fix for #1257, map download null.zip

pass the title of the map along to geoserver
add doc about this method and the readme field
Change priority of geoserver's package
Updated changelog for version 2.0c5
Remove "or groups" from the upload template until group permissions are supported.  Fixes #1262.