Engage: Fediverse layer — ActivityPub actor, milestones and opt-in ratings

[oesteban]

Make each DataPeer node an ActivityPub actor, so a federation has a presence on
the Fediverse and the people who contribute to a collection can be recognised
for it.

Scope:

• Actor document, followers collection, and delivery that POSTs each activity to
  every follower's inbox with retry on failure.
• HTTP Signatures over inbound and outbound activities. ActivityPub leaves
  authentication unspecified, so this is implemented rather than inherited.
• Milestone announcements: as a shared collection passes contribution
  thresholds, the node posts an update that anyone can follow from Mastodon. The
  tally is computed by the node from the collection it already holds, with no
  central platform counting for it.
• Opt-in, pseudonymous contributor board over the same collection.
• First instance: opt-in publishing of manual MRI quality-control ratings, which
  are already keyed by the rated image's content hash and so are ordinary
  tabular rows.

Operational note: an actor is named by an HTTPS URL and its inbox receives
ordinary HTTP POSTs, so this half of the node needs a public name, TLS and a
reverse proxy the operator provides. A node deployed without them federates data
normally and has no Fediverse presence.

Task 11 of the funded work plan (0.35 person-months).