Replies cause jumps during scrolling

### Steps to reproduce

Scroll through any room where replies are used frequently

### Outcome

#### What did you expect?

Smooth scrolling.

#### What happened instead?

Jumpy scrolling. This is somewhat unavoidable with the current reply layout, since reply previews are of variable height, meaning there's no way to predict how tall they will be before they load.

This could be solved by either giving reply previews a fixed height (would require an entirely new design), or preloading replies before they're ever paginated into the timeline.

### Operating system

NixOS unstable

### Browser information

Firefox 98.0.1

### URL for webapp

develop.element.io

### Application version

Element version: 02a5789dd362-react-af6bd63ac7ab-js-bdc3da1fac3c Olm version: 3.2.8

### Homeserver

Synapse 1.55.0

### Will you send logs?

No