Unknown events are not rendered, making it possible to miss important messages

[ichthyosaurus]

Is your feature request related to a problem? Please describe.
Example: when I receive voice messages / audio attachments, they don't show up in my messages list at all. I had some weird misunderstandings because of that already, when I suddenly missed part of a conversation. This applies to all current and future unknown events / message types, not only the case of audio messages that I already reported in #1187 and fixed in #1195 .

Describe the solution you'd like
Unsupported message types should show up in the timeline as a placeholder, so that I can at least know that there would be something that I can't see. Then I could check my messages with a different app, and ideally add an implementation in Hydrogen.