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/queue.py

@@ -1,57 +1,15 @@
-from flask_rq2 import RQ
-from flask import current_app as app
+from celery import Celery
+
+celery = Celery(__name__)
 
 
-class ATSTQueue(RQ):
+def update_celery(celery, app):
+    celery.conf.update(app.config)
 
-    """Internal helpers to get the queue that actually does the work.
+    class ContextTask(celery.Task):
+        def __call__(self, *args, **kwargs):
+            with app.app_context():
+                return self.run(*args, **kwargs)
 
-    The RQ object always uses the "default" queue, unless we explicitly request
-    otherwise. These helpers allow us to use `.queue_name` to get the name of
-    the configured queue and `_queue_job` will use the appropriate queue.
-
-    """
-
-    @property
-    def queue_name(self):
-        return self.queues[0]
-
-    def get_queue(self, name=None):
-        if not name:
-            name = self.queue_name
-        return super().get_queue(name)
-
-    def _queue_job(self, function, *args, **kwargs):
-        self.get_queue().enqueue(function, *args, **kwargs)
-
-    # pylint: disable=pointless-string-statement
-    """Instance methods to queue up application-specific jobs."""
-
-    def send_mail(self, recipients, subject, body):
-        self._queue_job(ATSTQueue._send_mail, recipients, subject, body)
-
-    def send_notification_mail(self, recipients, subject, body):
-        self._queue_job(ATSTQueue._send_notification_mail, recipients, subject, body)
-
-    # pylint: disable=pointless-string-statement
-    """Class methods to actually perform the work.
-
-    Must be a class method (or a module-level function) because we being able
-    to pickle the class is more effort than its worth.
-    """
-
-    @classmethod
-    def _send_mail(self, recipients, subject, body):
-        app.mailer.send(recipients, subject, body)
-
-    @classmethod
-    def _send_notification_mail(self, recipients, subject, body):
-        app.logger.info(
-            "Sending a notification to these recipients: {}\n\nSubject: {}\n\n{}".format(
-                recipients, subject, body
-            )
-        )
-        app.mailer.send(recipients, subject, body)
-
-
-queue = ATSTQueue()
+    celery.Task = ContextTask
+    return celery