Export doesn't show messages as threaded

### Steps to reproduce

1. Open a room with at least one thread in it
2. Click `Export chat`, select HTML, click `Export`.
3. Extract archive, open HTML file.

### Outcome

#### What did you expect?

I expect the messages that are part of a thread to be displayed as such.

#### What happened instead?

A clickable button (no action) with the text `x replies` is shown. The messages that happened in the thread are shown in the timeline where all other messages are shown, with no indication that they happened in the thread.

Source of the button:

```html

### Operating system

Ubuntu

### Application version

Element version: 1.11.31, Olm version: 3.2.14

### How did you install the app?

Ubuntu Software Center

### Homeserver

Synapse

### Will you send logs?

No