*Note: OStatus URIs are invalid for ActivityPub. But we have them for
as long as we want to keep old OStatus-sourced content and as long as
we remain OStatus-compatible.*
- In Announce handling, if object URI is not a URL, fallback to object URL
- Do not use specialized ThreadResolveWorker, rely on generalized handling
- When serializing notes, if parent's URI is not a URL, use parent's URL
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