Add and apply Rails/RedundantActiveRecordAllMethod

This rule was introduced in rubocop-rails 2.21.0.

spec/system/notifications_spec.rb

@@ -241,7 +241,7 @@ describe "Notifications" do
   end
 
   def users_without_notifications
-    User.all.select do |user|
+    User.select do |user|
       user.notifications.not_emailed.where(notifiable_type: "ProposalNotification").blank?
     end
   end