refactor(mailer): extract dolist logic into its concern

app/mailers/application_mailer.rb

@@ -1,4 +1,5 @@
 class ApplicationMailer < ActionMailer::Base
+  include MailerDolistConcern
   include MailerMonitoringConcern
 
   helper :application # gives access to all helpers defined within `application_helper`.

app/mailers/concerns/mailer_dolist_concern.rb

@@ -0,0 +1,15 @@
+module MailerDolistConcern
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :add_dolist_header
+
+    # mandatory for dolist
+    # used for tracking in Dolist UI
+    # the delivery_method is yet unknown (:balancer)
+    # so we add the dolist header for everyone
+    def add_dolist_header
+      headers['X-Dolist-Message-Name'] = action_name
+    end
+  end
+end

app/mailers/concerns/mailer_monitoring_concern.rb

@@ -2,8 +2,6 @@ module MailerMonitoringConcern
   extend ActiveSupport::Concern
 
   included do
-    before_action :add_dolist_header
-
     # Intercept & log any error, then re-raise so job will retry.
     # NOTE: rescue_from order matters, later matchers are tried first.
     rescue_from StandardError, with: :log_and_raise_delivery_error
@@ -21,14 +19,6 @@ module MailerMonitoringConcern
       end
     end
 
-    # mandatory for dolist
-    # used for tracking in Dolist UI
-    # the delivery_method is yet unknown (:balancer)
-    # so we add the dolist header for everyone
-    def add_dolist_header
-      headers['X-Dolist-Message-Name'] = action_name
-    end
-
     def log_and_raise_delivery_error(exception)
       EmailEvent.create_from_message!(message, status: "dispatch_error")
       Sentry.capture_exception(exception, extra: { to: message.to, subject: message.subject })

app/mailers/devise_user_mailer.rb

@@ -3,6 +3,7 @@ class DeviseUserMailer < Devise::Mailer
   helper :application # gives access to all helpers defined within `application_helper`.
   helper MailerHelper
   include Devise::Controllers::UrlHelpers # Optional. eg. `confirmation_url`
+  include MailerDolistConcern
   include MailerMonitoringConcern
   layout 'mailers/layout'
   before_action :add_delivery_method, if: :forced_delivery?