Support new mailgun's API key format

config/site_settings.yml

@@ -832,7 +832,7 @@ email:
     default: ''
   mailgun_api_key:
     default: ''
-    regex: '^key-\h{32}$'
+    regex: '^(key-\h{32}|\h{32}-\h{8}-\h{8})$'
   bounce_score_threshold:
     client: true
     default: 4