We add `Access-Control-Allow-Origin: *` to all asset requests which are requested via a configured CDN. This is particularly important now that we're using browser-native `import()` to load the highlightjs bundle. Unfortunately, user-configurable 'cors_origins' site setting was overriding the wldcard value on CDN assets and causing CORS errors.
This commit updates the logic to give the `*` value precedence, and adds a spec for the situation. It also invalidates the cache of hljs assets (because CDNs will have cached the bad Access-Control-Allow-Origin header).
The rack-cors middleware is also slightly tweaked so that it is always inserted. This makes things easier to test and more consistent.
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