Add ability to delete messages manually and/or automatically

Hmm... yeah, I guess right now this is only available to users w/ administer content privileges. I don't consider this an essential feature for the 1.0 release, but it's definitely a worthy feature request! The tough thing here is that PMs are a single node in the author's sent box and the recipient's inbox. Deleting really just needs to hide the node from one or the other's display until both delete the message or perhaps a certain time limit or message threshold has been reached. I'd like to see this specced out and included in a later version.

I have users too that are complaining that they can't delete messages, this also causes a problem not being able to delete messages when there are no page numbers, I would be fine with no page numbers if there was at least a delete feature because then I can just delete the old ones and they won't be in the way for me to look through the pages.

Pagers will be added in: #358352: Add pager to Inbox and Sent messages tables