Note deletion

[julianlam]

When remote objects are received, they are parsed and a local representation is saved to the database.

Occasionally, we may want to delete these notes (e.g. pruning, received deletion, etc.)

Existing NodeBB logic should be updated so that if an existing remote post is passed in, all additional references added by src/activitypub/notes.js are handled.

• When fetched via S2S, a Tombstone object should be served for deleted posts
• When local posts are deleted, an Update(Tombstone) should be federated out
• Likewise for post restoration
• Upon receipt of an Update(Tombstone), the local representation of the Note should be soft deleted
• When posts are purged, a Delete(Note) should be federated out
• Upon receipt of a Delete(Note), the Note should be re-retrieved. If 404/410, purge. If Tombstone, delete.

[julianlam]

Nomenclature-wise, an ActivityPub delete corresponds to a NodeBB purge. The local representation of that remote content is removed from the database completely. kaniini argues that you could treat it as a cache invalidation instead, and so re-fetch if able.

Re: Tombstones there's a rough parallel between serving a Tombstone on a soft delete, and a 404 after a purge.