It turns out to be quite expensive to update the state, because we can't do
shouldComponentUpdate on any of the sub-components (because RRs and local echo
sneak in through the back door), and we don't want to trigger a whole render
cycle every time someone presses a key.
Separate the read-up-to marker from the read-receipts, and show a status bar
when the read marker is above the screen. Move the read-marker down when it is
on the screen and there is user activity.
(This requires corresponding changes in vector-web, to provide the CSS and img)
Richard van der Hoff