Support new mailgun's API key format

Support new mailgun’s API key format

diff --git a/config/site_settings.yml b/config/site_settings.yml
index 3a6a967..22adacc 100644
--- a/config/site_settings.yml
+++ b/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

GitHub sha: 474a01f3

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