Add Juju action to easily clear mail queues

Hi,

Add a Juju action to do something like this:

| mailq | grep $sender | awk {'print $1'} | xargs -I{} sudo postsuper -d {}

Or more trickly with recipient since it's on it's own line:

| 1F12613B45E* 5602 Thu Aug 31 20:54:30 <email address hidden>
| <email address hidden>

The action should include a "dry-run" mode to show all the messages matched.

Perhaps even an action to `postcat -vq` to inspect a message in the queue?