swh-journal: Complete missing snapshot insertion event from storage to journal

Most of the events are installed except for the snapshot (which came later).

Adaptations needed:

install the current trigger in the storage db
update the storage listener to subscribe (storage subscriber) to this event and push the identifier in the kafka topic (kafka producer) [2]
update the reifier layer which as a first approximation will fetch those new id and reify naively the full snapshot (some are huge) [3]
[1] sql/70-swh-trigger.sql
[2] swh.storage.listener
[3] swh.journal.publisher

Migrated from T1275 (view on Phabricator)

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