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

app/models/user.rb

@@ -170,6 +170,10 @@ class User < ApplicationRecord
     settings.default_privacy || (account.locked? ? 'private' : 'public')
   end
 
+  def allows_digest_emails?
+    settings.notification_emails['digest']
+  end
+
   def token_for_app(a)
     return nil if a.nil? || a.owner != self
     Doorkeeper::AccessToken