Fix attribute form on layer metadata page

Currently the attribute form on the layer metadata page is always invalid (required fields are not excluded in the form) so any edits to the attributes are never saved.

- Added several fields to the form (display_order and attribute_label)
- Added other required fields not on the form to the excluded list (min, max, average, etc)
- Added validation/error message handling to the template

Fix for issue https://github.com/GeoNode/geonode/issues/1037