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

spec/controllers/admin/instances_controller_spec.rb

@@ -9,10 +9,10 @@ RSpec.describe Admin::InstancesController, type: :controller do
 
   describe 'GET #index' do
     around do |example|
-      default_per_page = Account.default_per_page
-      Account.paginates_per 1
+      default_per_page = Instance.default_per_page
+      Instance.paginates_per 1
       example.run
-      Account.paginates_per default_per_page
+      Instance.paginates_per default_per_page
     end
 
     it 'renders instances' do