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

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.

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