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}!
-    %p
-      The password for your GitLab account on
-      #{link_to(Gitlab.config.gitlab.url, Gitlab.config.gitlab.url)}
-      has successfully been changed.
-    %p
-      If you did not initiate this change, please contact your administrator
-      immediately.
+= 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)}
+    has successfully been changed.
+  %p
+    If you did not initiate this change, please contact your administrator
+    immediately.

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

-.center
-  #content
-    %h2 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)}.
-    %p
-      If you did not perform this request, you can safely ignore this email.
-    %p
-      Otherwise, click the link below to complete the process.
-    #cta
-      = link_to('Reset password', edit_password_url(@resource, reset_password_token: @token))
+= 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)}.
+  %p
+    If you did not perform this request, you can safely ignore this email.
+  %p
+    Otherwise, click the link below to complete the process.
+  #cta
+    = link_to('Reset password', edit_password_url(@resource, reset_password_token: @token))