Local echo redactions are misleading

Dendrite did not implement the `/redact` endpoint correctly, so requests would 404 to it. However, Element would local echo redact the message, and then say things like "some messages were not sent" when there are no messages to send on the UI. Ideally the redaction text would use red/grey/white signals like normal messages do.

As a result of this misleading UI, it took us (Dendrite folks) a while to work out that there was nothing wrong with redactions over federation or even on the server, but that it was just a missing CS API endpoint. Worth noting that Sytest uses a deprecated nonspec version of /redact which is what we actually implemented, hence the disparity.