Mastodon nginx: fix CORS on /instance

installation/mastodon.conf

@@ -104,6 +104,11 @@ server {
   # Enable CORS so we can fetch them.
   location /instance {
     add_header Access-Control-Allow-Origin "*";
+
+    if ($request_method = 'OPTIONS') {
+      add_header Access-Control-Allow-Origin "*";
+      return 204;
+    }
   }
 
   # Soapbox ServiceWorker.