Store information about pinned messages #33

Right now statbot does not track any information about pins because of some difficulty in doing so. When a pin is created, a " pinned a message." system message appears. This provides information on who and when the pinning occurred. It, however, does not say which message was pinned. This is normally something you get by looking at the list of pinned messages, and seeing which one came last.

But the problem is that you can delete those system messages, and when you unpin messages, those system messages remain. This makes matching up information about who pinned a message with the message that was pinned nontrivial, and in cases of deleted messages, sometimes impossible.