Collection changes and migration for federation

- Adds a "type" column to the Collection object and allows the
CollectionItem model to contain any object.
- Changes "items" to "num_items" as per TODO
- Renames "uploader", "creator" and "user" to a common "actor" in most places

mediagoblin/gmg_commands/batchaddmedia.py

@@ -213,4 +213,3 @@ def parse_csv_file(file_contents):
         objects_dict[media_id] = (line_dict)
 
     return objects_dict
-