Don't show unknown/custom state events in rooms by default

[tulir]

Your use case

What would you like to do?

Stop users from seeing confusing "custom events"

Why would you like to do it?

Many users don't know how Matrix works, and some users have strong adverse reactions to anything unexpected in their chat view

How would you like to achieve it?

Don't show any unknown event types in the room timeline, except if the user opts into viewing hidden events via a developer option (#3760)

Have you considered any alternatives?

Users could be educated to understand Matrix better and not get scared about unknown things, but that is likely more difficult than hiding unknown events

Additional context

No response

Are you willing to provide a PR?

No