Move e-mail digest task to sidekiq, reduce workload, improve hint (#6252)

app/workers/digest_mailer_worker.rb

@@ -9,7 +9,7 @@ class DigestMailerWorker
 
   def perform(user_id)
     @user = User.find(user_id)
-    deliver_digest if user_receives_digest?
+    deliver_digest if @user.allows_digest_emails?
   end
 
   private
@@ -18,8 +18,4 @@ class DigestMailerWorker
     NotificationMailer.digest(user.account).deliver_now!
     user.touch(:last_emailed_at)
   end
-
-  def user_receives_digest?
-    user.settings.notification_emails['digest']
-  end
 end