Signed messages in unencrypted rooms

Due to regulatory, administrative and other reasons, there will be rooms or even whole homeservers with disabled E2E encryption. But that doesn't mean we should drop related security features like cryptographical identity and message signing.

We can reuse existing session key management mechanisms to provide security against homeserver/federation-side tampering and client session compromising, by conditionally switching AES-CBC with no-op.

This issue extends from #212

Start by reading the related issue #212 and the existing session key management mechanisms referenced here. Define the protocol changes needed to preserve cryptographic identity and message signing in unencrypted rooms, including the proposed conditional replacement of AES-CBC. Done should be a concrete, reviewed specification for the behavior and security model.

Written by the indexing model from the issue text.