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

db/migrate/20201206004238_create_instances.rb

@@ -0,0 +1,9 @@
+class CreateInstances < ActiveRecord::Migration[5.2]
+  def change
+    create_view :instances, materialized: true
+
+    # To be able to refresh the view concurrently,
+    # at least one unique index is required
+    safety_assured { add_index :instances, :domain, unique: true }
+  end
+end