Fix slow distinct queries where grouped queries are faster (#15287)

About 2x speed-up on inboxes query

app/models/form/account_batch.rb

@@ -51,7 +51,7 @@ class Form::AccountBatch
   end
 
   def account_domains
-    accounts.pluck(Arel.sql('distinct domain')).compact
+    accounts.group(:domain).pluck(:domain).compact
   end
 
   def accounts