Highlight message if we miss some of references and in-reply-to context

[ian]

Clearly mark it on the interface when a received message references or is in-reply-to another message that our client does not know about.

One possibility is:

• draw a dim message bubble above it,
• mention the number of referenced messages missing
• format the received message as if it was a reply to this one

This would improve disambiguation and debugging federation issues at the same time.

Related:

Transcript consistency by referencing message hashes in context and backfill

Generate and account a content-addressable ID for each message - a cryptographic hash of the content, metadata and important headers of said message (see DKIM for inspiration). It would be useful to store it in the message ID, but that’s not a requirement as it’s implicit. In case we did, a new header could be introduced that reliably signaled how the message ID field was generated (at a minimum, as an algorithm version number, otherwise via elaborate DKIM-like headers). Always reference the ab…