Merge branch 'move-mailer-prepends' into 'master'

Move EE prepend/include to the end of mailers

See merge request gitlab-org/gitlab-ee!8202

app/mailers/previews/notify_preview.rb

 # frozen_string_literal: true
 
 class NotifyPreview < ActionMailer::Preview
-  prepend EE::Preview::NotifyPreview
-
   def note_merge_request_email_for_individual_note
     note_email(:note_merge_request_email) do
       note = <<-MD.strip_heredoc
@@ -196,3 +194,5 @@ class NotifyPreview < ActionMailer::Preview
     email
   end
 end
+
+NotifyPreview.prepend(EE::Preview::NotifyPreview)