Mailman3: add UI setting

roles/mailman/templates/settings.py.j2

@@ -286,6 +286,12 @@ COMPRESS_OFFLINE = True
 CRISPY_TEMPLATE_PACK = 'bootstrap3'
 CRISPY_FAIL_SILENTLY = not DEBUG
 
+# Compatibility with Bootstrap 3
+from django.contrib.messages import constants as messages
+MESSAGE_TAGS = {
+    messages.ERROR: 'danger'
+    }
+
 
 #
 # Full-text search engine
@@ -371,7 +377,6 @@ CACHES = {
 }
 
 
-
 #
 # HyperKitty-specific
 #