Generate valid Message-ID in email rejection mailer

Use a Message-ID that is RFC 2111 compliant. This fix is consistent with
how the Message-ID is generated in the 'notify' mailer.

app/mailers/email_rejection_mailer.rb

@@ -10,7 +10,7 @@ class EmailRejectionMailer < BaseMailer
       subject: "[Rejected] #{@original_message.subject}"
     }
 
-    headers['Message-ID'] = SecureRandom.hex
+    headers['Message-ID'] = "<#{SecureRandom.hex}@#{Gitlab.config.gitlab.host}>"
     headers['In-Reply-To'] = @original_message.message_id
     headers['References'] = @original_message.message_id