Followers/following collections

[srosset81]

https://www.w3.org/TR/activitypub/#followers
https://www.w3.org/TR/activitypub/#following

Followers/following collections are not required but are very used in the fediverse.

• Attach followers/following AS collections to the WebID
• Handle side effects
  • When receiving or emitting Follow activities, add the corresponding items to the followers/following AS collections. (ref.)
  • Eventually send back an Accept activity