Merge branch 'use_helper_method' into 'master'

Use method helper instead of add_template_helper

On my opinion, better to use method `helper` instead of `add_template_helper`.
Maybe there is reason doing it here. could you please explain if yes?
Thanks!

See merge request !7315

app/mailers/base_mailer.rb

 class BaseMailer < ActionMailer::Base
-  add_template_helper ApplicationHelper
-  add_template_helper GitlabMarkdownHelper
+  helper ApplicationHelper
+  helper GitlabMarkdownHelper
 
   attr_accessor :current_user
   helper_method :current_user, :can?

app/mailers/notify.rb

@@ -10,12 +10,12 @@ class Notify < BaseMailer
   include Emails::Pipelines
   include Emails::Members
 
-  add_template_helper MergeRequestsHelper
-  add_template_helper DiffHelper
-  add_template_helper BlobHelper
-  add_template_helper EmailsHelper
-  add_template_helper MembersHelper
-  add_template_helper GitlabRoutingHelper
+  helper MergeRequestsHelper
+  helper DiffHelper
+  helper BlobHelper
+  helper EmailsHelper
+  helper MembersHelper
+  helper GitlabRoutingHelper
 
   def test_email(recipient_email, subject, body)
     mail(to: recipient_email,