Move unreadMessageElement from computed to a method

Depending on invocation order otherwise the computed
could be first used before the element was mounted to the DOM.
That would cache the computed and cause the element
to never exist although it was added to the DOM afterwards.

The problem with a cached "null" is that
"first unread message has not been seen yet, so don't move it"
then always exited the updating of the readmarker.

Signed-off-by: Joas Schilling <coding@schilljs.com>