Support rendering of extensible events v2

[turt2live]

Requires https://github.com/matrix-org/matrix-js-sdk/pull/3067

See https://github.com/matrix-org/matrix-js-sdk/pull/3067 for details.

This looks and acts horribly broken because, well, it is:

Known issues:

• Messages look like that
• Sending a message causes a tile crash
• These events cannot be reacted to, replied to, made a thread from, etc
• etc

None of these issues are fixed in this PR, deliberately. The whole feature is gated by a room version, which means you also get this lovely warning at the top of the room you can't get rid of:

This should be enough to consider the room version similar to a labs flag, thus making the bugs expected/reasonable.

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Here's what your changelog entry will look like:

✨ Features

• Support rendering of extensible events v2 (#9915).