Sending a PM should invalidate the PM dropdown cache

[Colonel_mortis]

If you open the PM dropdown to view previous sent PMs, then send a PM using the Message button on a user's hovercard, the PM dropdown cache is not invalidated, so the user can't review the sent PM without refreshing the page or clicking "go to inbox", which, in the case that triggered this, led the user to think that they couldn't see the PM at all until the other user replied. If you listened for the submit event on the PM submit form, and ran

$(document).trigger("clearUserbarCache", {type:'inbox'});

that would make it reload next time the messages dropdown is opened, leading to a better UX.