Initial commit to support Celery 4.3.0

Merges 55e9b2263c from versions/next
with code from GitLab issue #594 and GitLab merge request !55.

Thanks to Jakob Haufe (@sur5r) and Jesaja Everling (@jeverling)
for much of the research and code updates.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/celery.py

@@ -9,6 +9,5 @@ from .runtime import celery_class
 os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'mayan.settings.production')
 
 app = celery_class('mayan')
-
-app.config_from_object('django.conf:settings')
+app.config_from_object('django.conf:settings', namespace='CELERY')
 app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)