For mailing-list messages:
- convert all text-only messages to XHTML for consistency.
- detect and flag "noisy" content, i.e. content which are not useful and annoying in
forum-like view (full-lenght quoted previous messages, mailing-list signature)
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Pydantic models are used more and more in Libervia, for the bridge API, and also to
convert `domish.Element` to internal representation.
Type hints have also been added in many places.
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