Support for `attribute_set` for detailed description of the dataset

[ridoo]

Support for attributes_set for detailed description of the dataset with the help of metadata: the columns of a table should be set as “attributes_set” (GeoNode description of columns and labels). If necessary, also transfer information regarding description from datapackage.json.

• make attributes_set available via metadata editor and API
• parse and set attribute metadata declared in datapackage.json