use common layout for devise mailer

app/mailers/devise_mailer.rb

@@ -2,7 +2,9 @@ class DeviseMailer < Devise::Mailer
   default from: "#{Gitlab.config.gitlab.email_display_name} <#{Gitlab.config.gitlab.email_from}>"
   default reply_to: Gitlab.config.gitlab.email_reply_to
 
-  layout 'devise_mailer'
+  layout 'mailer'
+
+  helper EmailsHelper
 
   protected
 

app/views/devise/mailer/password_change.html.haml

-.center
-  #content
-    %h2 Hello, #{@resource.name}!
+= render layout: 'layouts/mailer/default_content' do
+  Hello, #{@resource.name}!
   %p
     The password for your GitLab account on
     #{link_to(Gitlab.config.gitlab.url, Gitlab.config.gitlab.url)}

app/views/devise/mailer/reset_password_instructions.html.haml

-.center
-  #content
-    %h2 Hello, #{@resource.name}!
+= render layout: 'layouts/mailer/default_content' do
+  Hello, #{@resource.name}!
   %p
     Someone, hopefully you, has requested to reset the password for your
     GitLab account on #{link_to(Gitlab.config.gitlab.url, Gitlab.config.gitlab.url)}.