FIX: Delete push subscription on MismatchSenderId error

This should prevent repeated MismatchSenderId warnings when the VAPID keys have changed.

app/services/push_notification_pusher.rb

@@ -88,6 +88,9 @@ class PushNotificationPusher
     rescue Webpush::ExpiredSubscription
       unsubscribe(user, subscription)
     rescue Webpush::ResponseError => e
+      if e.response.message == "MismatchSenderId"
+        unsubscribe(user, subscription)
+      else
         Discourse.warn_exception(
           e,
           message: "Failed to send push notification",
@@ -100,3 +103,4 @@ class PushNotificationPusher
       end
     end
   end
+end