fix #2073fix#2292 (I think)
- Messages can be assigned to any chat by the References and In-Reply-To, also to 1:1 chats; this has higher priority than the group id because with ad-hoc groups, it can happen that two devices have different group ids for the same conversation thread.
- If `From` is not in the chat (we call this "shadow sender"), `OverrideSenderDisplayname` is set. This communicates to the UI that:
- A `~`should be added in front of the sender's displayname.
- Also in 1:1 chats, the sender's displayname and avatar is shown, as if this was a group.
The "Unknown sender for this chat" messages are completely removed for unprotected groups.
For protected chats, everything stays as it was before.
POSTPONED:
- Maybe (if it turns out to be still necessary):
- The ad-hoc group id is computed by the the References, instead of the member list, as it is currently done
- How do we prevent that the message can't be assigned to the correct chat as the parent message was deleted?
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