queue: Monitor user_activity queue, now that it has a consumer.

Since this was using repead individual get() calls previously, it
could not be monitored for having a consumer.  Add it in, by marking
it of queue type "consumer" (the default), and adding Nagios lines for
it.

Also adjust missedmessage_emails to be monitored; it stopped using
LoopQueueProcessingWorker in 5cec566cb9, but was never added back
into the set of monitored consumers.

zerver/worker/queue_processors.py

@@ -434,7 +434,7 @@ class ConfirmationEmailWorker(QueueProcessingWorker):
                 context=context,
                 delay=datetime.timedelta(days=settings.INVITATION_LINK_VALIDITY_DAYS - 2))
 
-@assign_queue('user_activity', queue_type="loop")
+@assign_queue('user_activity')
 class UserActivityWorker(LoopQueueProcessingWorker):
     """The UserActivity queue is perhaps our highest-traffic queue, and
     requires some care to ensure it performs adequately.
@@ -515,7 +515,7 @@ class UserPresenceWorker(QueueProcessingWorker):
         status = event["status"]
         do_update_user_presence(user_profile, client, log_time, status)
 
-@assign_queue('missedmessage_emails', queue_type="loop")
+@assign_queue('missedmessage_emails')
 class MissedMessageWorker(QueueProcessingWorker):
     # Aggregate all messages received over the last BATCH_DURATION
     # seconds to let someone finish sending a batch of messages and/or