Export chat function freezes Element web

### Steps to reproduce

On both Firefox and Chromium. On a private, e2e encrypted room with 89000 events, try the "export chat function", with "From the beginning", size 8 MBs, include attachments no.

Everything goes fine until creating the html. It also never uses my full system memory, it only consumes 85%, at around 12 Gbs.

Then, the browser screen goes white on Firefox. Or on Chrome, I get a "something wrong has happened".

Excerpt logs from `/rageshake`:
```
I Creating HTML...
2022-03-12T14:52:15.556Z W Possible EventEmitter memory leak detected. 101 Thread.new listeners added. Use emitter.setMaxListeners() to increase limit
d@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:8597
s.prototype.addListener@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:11181
componentDidMount@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:549189
hs@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4153443
Tc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4170501
t.unstable_runWithPriority@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4195787
Vi@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4112378
Cc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4167048
hc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4163157
sc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4160280
Kc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4176600
Zc/<@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4178022
mc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4163310
Zc@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4178007
t.render@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:4185455
getEventTileMarkup@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:6706538

(...)

Then around 50 of these:

2022-03-12T14:56:50.912Z E this.props.callEventGrouper is undefined
Bi@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:6858498
a@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:6720735
xe@https://app.element.io/bundles/069fd4d72f624a5a90be/vendors~init.js:2:6721931
Ce