FIX: setup permission check based on group membership.

former permission check, based on django model permission won't work,
because we have no model, that would create the permission.

dav_submission/apps.py

@@ -13,6 +13,7 @@ DEFAULT_SETTINGS = (
     DefaultSetting('max_total_size_mib', 100),
     DefaultSetting('metadata_file_name', 'metadata.txt'),
     DefaultSetting('cached_zip_file_name', '.cache.zip'),
+    DefaultSetting('download_group', 'downloaders'),
 )