Added MailHeaderMiddleware to settings.py for mailman-web

Added the MailHeaderMiddleware to settings.py for the mailman-web
service.

comanage-registry-mailman/web/mailman-web/settings.py

@@ -111,6 +111,7 @@
     'django.middleware.csrf.CsrfViewMiddleware',
     'django.middleware.locale.LocaleMiddleware',
     'django.contrib.auth.middleware.AuthenticationMiddleware',
+    'django_mail_header.middleware.MailHeaderMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
     'django.middleware.clickjacking.XFrameOptionsMiddleware',
     'django.middleware.security.SecurityMiddleware',