Fix post history appearing blank when joining a new group via backfill delivery

[wmurphyrd]

An issue that affects all ActivityPub implementations is that, when you first follow someone, you may not have their post history available if no-one else on your server was following them then they made the posts. This is especially limiting for Guppe because the post history is not currently made available on the Web interface, so there's no other way to see it.

I propose we solve this with a new feature: backfill delivery

After accepting a follow request, the server will pull up some recent history from the group and deliver it (message-by-message) to that new follower. This will cause the profile page to fill up if their server did not already have the history. If they did already have the history, compliant servers will ignore the duplicate deliveries.

Estimate: 4 hours

[moeria]

This request should be a priority, please. It's really completely absurd not to see any post! Because then what are groups for? Cheers from a user who wants to strengthen the concern.