cors errors when running examples in browser

[ryanleecode]

Access to fetch at 'http://localhost:4646/frame/dev/polkadot/@v0.9.37/user_i' from origin 'null' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
VM25:2942          POST http://localhost:4646/frame/dev/polkadot/@v0.9.37/user_i net::ERR_FAILED
users2 @ VM25:2942
(anonymous) @ VM25:21410
VM20:2728 TypeError: Failed to fetch
    at users2 (<anonymous>:2942:28)
    at <anonymous>:21410:21

[tjjfvi]

Ah yes, we need to add the proper cors headers to all requests returned by the local server, as well as the two capi.dev servers.

Probably best solved with a handleCors wrapper a la handleErrors.