Bluesky: filter out unfollows from follow spammers

[snarfed]

Right now, unfollows on Bluesky from (afaict) follow spammers make up the majority of all activities that we handle. Ugh.

I'm going to disable the biggest ones manually, but I'd like a better way to detect and prevent or handle this. 😐

Examples:

• https://fed.brid.gy/bsky/did:plc:b3sqpvipguta66xwpxuyf2mh
• https://fed.brid.gy/bsky/did:plc:umheuxjx6xmj4qptlvgtbwtq
• https://fed.brid.gy/bsky/did:plc:rohqb56guqhs66i5fmtazhts
• https://fed.brid.gy/bsky/did:plc:6uonzixencenrs4r6wvv5buf
• https://fed.brid.gy/bsky/did:plc:4rsec7snwsap5hzp3prgkhkn
• https://fed.brid.gy/bsky/did:plc:yd3vun73elmxpevhgf4l6aby

[snarfed]

I'd like a better way to detect and prevent or handle this. 😐

One option would be to subscribe to the Bluesky team's mod service and disable accounts that get labelled as spam. Related to #971.

[snarfed]

Hacked together an approximation ^, good enough for now