FEATURE: Cache CORS preflight requests for 2h (#14614)

* FEATURE: Cache CORS preflight requests for 2h

Browsers will cache this for 5 seconds by default. If using MessageBus
in a different domain, Discourse will issue a new long polling, by
default, every 30s or so. This means we would be issuing a new preflight
request **every time**. This can be incredibly wasteful, so let's cache
the authorization in the client for 2h, which is the maximum Chromium
allows us as of today.

* fix tests

config/initializers/008-rack-cors.rb

@@ -45,6 +45,7 @@ class Discourse::Cors
       headers['Access-Control-Allow-Headers'] = 'Content-Type, Cache-Control, X-Requested-With, X-CSRF-Token, Discourse-Present, User-Api-Key, User-Api-Client-Id, Authorization'
       headers['Access-Control-Allow-Credentials'] = 'true'
       headers['Access-Control-Allow-Methods'] = 'POST, PUT, GET, OPTIONS, DELETE'
+      headers['Access-Control-Max-Age'] = '7200'
     end
 
     headers