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

Gemfile.lock

@@ -561,6 +561,9 @@ GEM
       crass (~> 1.0.2)
       nokogiri (>= 1.8.0)
       nokogumbo (~> 2.0)
+    scenic (1.5.4)
+      activerecord (>= 4.0.0)
+      railties (>= 4.0.0)
     securecompare (1.0.0)
     semantic_range (2.3.0)
     sidekiq (6.1.2)
@@ -782,6 +785,7 @@ DEPENDENCIES
   rubocop-rails (~> 2.8)
   ruby-progressbar (~> 1.10)
   sanitize (~> 5.2)
+  scenic (~> 1.5)
   sidekiq (~> 6.1)
   sidekiq-bulk (~> 0.2.0)
   sidekiq-scheduler (~> 3.0)