Element X: Return an error when sharing a room key to a verified user, who is no longer verified

[richvdh]

Part of element-hq/element-meta#2488, and a follow-up to #1129 and #3792.

We need to prevent people who have explicitly verified someone accidentally sending messages. OlmMachine::share_room_key should check whether any of the target users have previously been verified; if so, and those users are no longer verified, share_room_key should return an error code.

We should ensure the error code is propagated up through the ui crate.

This behaviour needs to be optional, so should be configured via EncryptionSettings (could be the same setting as #3792).

[richvdh]

See element-hq/element-meta#2488 (comment) for designs

[pixlwave]

For my reference:

For "verification violation" there is a new UserIdentity::withdraw_verification that you can call, then retry sending. (you can also blacklist if you prefer via Device::set_local_trust)
⁦And same the error gaves you all the problematic user_id, so you can just iterate, withdraw_verification then resend