Allow message sender to suggest disabling URL previews

It would be nice if the message sender could suggest whether or not to show URL previews. There could still be a way to force-show the preview if the user really wants it, but by default the sender's suggestion should be respected. Some bots send messages with lots of links and showing previews for any of them would unnecessarily fill up the room.

In practice, either [MSC2376](https://github.com/matrix-org/matrix-doc/pull/2376) or [MSC2385](https://github.com/matrix-org/matrix-doc/pull/2385) should be implemented for incoming messages, then maybe add some UI that users sending messages can use to disable the preview.

See also: https://github.com/vector-im/element-android/pull/2490#issuecomment-739926226