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/policies/domain_block_policy.rb

@@ -13,6 +13,10 @@ class DomainBlockPolicy < ApplicationPolicy
     admin?
   end
 
+  def update?
+    admin?
+  end
+
   def destroy?
     admin?
   end