Use Celery instead of RQ.

Celery provides a more robust set of queueing options for both tasks and
worker processes. Updates include:
- infrastructure necessary to run Celery, including celery entrypoint
- backgrounded functions are now imported directly from atst.jobs
- update tests as-needed
- update kubernetes worker pod command

atst/utils/notification_sender.py

@@ -1,6 +1,6 @@
 from sqlalchemy import select
 
-from atst.queue import ATSTQueue
+from atst.jobs import send_notification_mail
 from atst.database import db
 from atst.models import NotificationRecipient
 
@@ -8,12 +8,9 @@ from atst.models import NotificationRecipient
 class NotificationSender(object):
     EMAIL_SUBJECT = "ATST notification"
 
-    def __init__(self, queue: ATSTQueue):
-        self.queue = queue
-
     def send(self, body, type_=None):
         recipients = self._get_recipients(type_)
-        self.queue.send_notification_mail(recipients, self.EMAIL_SUBJECT, body)
+        send_notification_mail.delay(recipients, self.EMAIL_SUBJECT, body)
 
     def _get_recipients(self, type_):
         query = select([NotificationRecipient.email])