emacs: break out thread navigation from notmuch-show-archive-thread

This function is now just for archiving the current thread.  A new
function is created to archive-then-next.  The 'a' key binding is
updated accordingly.

This will allow people to bind to the simple thread archiving function
without the extra navigation.  The archive-thread function now also
takes a prefix to unarchive the current thread (ie. put the whole
thread back in the inbox).