component AP gateway: convert AP mention to XEP-0372 mentions:
when a mentions are found in AP items (either with people specified directly as target, of
with `mention` tags as in
https://www.w3.org/TR/activitystreams-vocabulary/#microsyntaxes), they are converted to
XEP-0372 `mention` references.
This is only done for Pubsub items (i.e. not for private messages).
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