Merge pull request #2005 from loic/ticket21169

Use 'update_fields' in RelatedManager.clear() when bulk=False.
Fixed a deprecation warning introduced by 96dd48c83f.
Fixed #21380 -- Added a way to set different permission for static directories.

Previously when collecting static files, the directories would receive permissions
from the global umask. Now the default permission comes from FILE_UPLOAD_DIRECTORY_PERMISSIONS
and there's an option to specify the permissions by subclassing any of the
static files storage classes and setting the directory_permissions_mode parameter.
Fixed #21323 -- Improved readability of serialized Operation.