UI for verifiable forwarded events

[tulir]

This implements the client side of MSC2730: sending forwards with the new endpoint and rendering validated forwards properly.

For sending forwards, the old forward button is replaced. The rendering is currently a very basic reply-like rendering, but it should probably be more distinct based on the designs in vector-im/element-web#14690. I might implement the better designs in this PR or a future one.

MSC: matrix-org/matrix-doc#2730 Synapse PR: matrix-org/synapse#8078 matrix-js-sdk PR: matrix-org/matrix-js-sdk#1439

Signed-off-by: Tulir Asokan <[email protected]>

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Here's what your changelog entry will look like:

✨ Features

• UI for verifiable forwarded events (#5117). Contributed by @tulir.