Fix performance on instances list in admin UI (#15282)

- Reduce duplicate queries
- Remove n+1 queries
- Add accounts count to detailed view
- Add separate action log entry for updating existing domain blocks

app/controllers/api/v1/instances/peers_controller.rb

@@ -8,7 +8,7 @@ class Api::V1::Instances::PeersController < Api::BaseController
 
   def index
     expires_in 1.day, public: true
-    render_with_cache(expires_in: 1.day) { Account.remote.domains }
+    render_with_cache(expires_in: 1.day) { Instance.where.not(domain: DomainBlock.select(:domain)).pluck(:domain) }
   end
 
   private