notmuch.el: Fix notmuch-snow-next-message when on the last message.

notmuch.el: Fix notmuch-snow-next-message when on the last message.

Before this just brought the current line to the top of the
window. Now it actually moves to the beginning of the current message.

This is built on a much more solid foundation now with a function to
move to the summary-line of the current message, and then moving from
there.