improve queue management

- add option to put messages in the queue "on hold", preventing delivery
  attempts until taken off hold again.
- add "hold rules", to automatically mark some/all submitted messages as "on
  hold", e.g. from a specific account or to a specific domain.
- add operation to "fail" a message, causing a DSN to be delivered to the
  sender. previously we could only drop a message from the queue.
- update admin page & add new cli tools for these operations, with new
  filtering rules for selecting the messages to operate on. in the admin
  interface, add filtering and checkboxes to select a set of messages to operate
  on.

prometheus.rules

@@ -28,6 +28,12 @@ groups:
     annotations:
       summary: http 5xx responses from webserver
 
+  - alert: mox-queue-hold
+    expr: mox_queue_hold > 0
+    for: 2h
+    annotations:
+      summary: messages on hold in queue for at least two hours
+
   - alert: mox-submission-errors
     expr: increase(mox_smtpserver_submission_total{result=~".*error"}[1h]) > 0
     annotations: