Remove reliance on DOM API to generated message preview

[germain-gg]

Improves element-hq/element-web#14750

When previews are enabled HTML messages rely on the DOM to extract the text from the event.
We can drop our reliance on the DOM and use sanitizeHtml only.

The important part is to allow no tags and keep disallowedTagsMode: 'discard'

If you set disallowedTagsMode to discard (the default), disallowed tags are discarded. Any text content or subtags is still included, depending on whether the individual subtags are allowed.

This can take up to ~2.5ms per room tile rendered on screen from the benchmark I have seen and we perform a double HTML parsing.

1. The first one by sanitizeHtml
2. Switfly followed by an .innerHtml call

[jryans]

Thanks, looks reasonable to me! 😄 What does the performance look like after the change?

[germain-gg]

It pretty much halves the above. You can see that for every sanitizeHtml call there's two "blue square". Removing the innerHtml call gets rid of one of those blue rectangles and the waiting time associated with it