We now add the htmlMfm key when relevant, store this in the database, and we see it when fetching using e.g.
curl -L -H 'Accept: application/activity+json' "$ap_id"
The `@context` of the Activity Pub message now also contains `htmlMfm: https://w3id.org/fep/c16b#htmlMfm`.
When an incomming post has `htmlMfm: true`, we will not re-parse the content.
FEDERATION.md is adapted to show the `htmlMfm` term is used.
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