Scrolling up in the timeline can cause the scroll position to jump when loading more messages

We should do something like [BACAT](https://github.com/matrix-org/matrix-react-sdk/blob/develop/docs/scrolling.md#bacat-bottom-aligned-clipped-at-top-scrolling). In any case, the timeline should be aligned to the bottom independent of the fact that there are enough tiles to fill the viewport or not.