Fix bug with date separator flashing up on scrollback
Fix bug with date separator flashing up on scrollback

Refactor the event-tile generation loop to go forwards rather than backwards,
which makes it easier to figure out whether we are displaying a continuation of
the previous event, and whether we need a date separator.

Also only display the date separator at the top of the room if there's no more
scrollback to be shown.

This fixes vector-im/vector-web#431

aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk