Add a preview email method

Suggested by @vij

Follows
https://docs.gitlab.com/ee/development/emails.html#mailer-previews

app/mailers/previews/notify_preview.rb

@@ -84,6 +84,10 @@ class NotifyPreview < ActionMailer::Preview
     Notify.issues_csv_email(user, project, '1997,Ford,E350', { truncated: false, rows_expected: 3, rows_written: 3 }).message
   end
 
+  def new_merge_request_email
+    Notify.new_merge_request_email(user.id, merge_request.id).message
+  end
+
   def closed_merge_request_email
     Notify.closed_merge_request_email(user.id, issue.id, user.id).message
   end