ActivityPub Utils: Greatly speed up the follow / block activity fetching.
Notifications in MastoAPI were unusably slow for me. The reason was that the fetching of the latest follow did not use our indexes and was very slow, which got triggered now in our notifications because we added the relationship by default as a help for PleromaFE.
This MR brought the query times from ~4s to 0.2ms (!) on soykaf.