Email improvement: Expose message queue for checking status and clearing

[d19dotca]

I'd like to request that we expose the message queue. Most dedicated mail server software allows for the ability to view the outgoing queue and clear the queue or individual messages too.

Having this ability means bounce backs to invalid email addresses caused by spam can be cleared out instead of being tried repeatedly for days, as just one example.

Admittedly this will be more useful for those running Cloudron with very active mail server usage which I suppose is in the minority of users, but would still be nice to have for those who are heavily relying on the mail server in Cloudron.

[d19dotca]

Looks like Haraka has a built-in web interface that could be a possible starting point here: http://haraka.github.io/plugins/watch/