Handle comet send-down in partialUpdate.

We now pull jsToAppend handling out of the message
handler for comet-context AJAX request processing all the
way to the wrapper around handling all messages. This
ensures that things like partial updates will properly render
their JS to append.

It also seems to incidentally fix S.jsToAppend within comet
rendering, which seemed to trigger the JS twice instead of
just once before this.