[Desktop] Use fallback mirrors for China users

[huchenlei]

Users in China have poor access to the mirrors, but sometimes the initial accessibility check can pass so the default mirror is selected.

This PR checks whether the user in China in a very reliable way and directly set the fallback mirrors for them.

┆Issue is synchronized with this Notion page by Unito

[huchenlei]

It seems like the check is not working because of cors policy. Need to find a workaround.

[webfiltered]

It might be good to (simultaneously) check download speed on a smallish file. Use the electron API to initiate this, and bypass all the CORS / browser limitations.

[robinjhuang]

Wonder if we should consider using the IP. Eg check the country code in the response of this API:
https://ipapi.co/json

[huchenlei]

Wonder if we should consider using the IP. Eg check the country code in the response of this API: https://ipapi.co/json

I get following response lol:

{
  "error": true,
  "reason": "RateLimited",
  "message": "Visit https://ipapi.co/ratelimited/ for details"
}

[huchenlei]

I think if we are not checking the response content like status, the opaque response is good enough.

[robinjhuang]

Surprised we got rate limited so fast. Supposed to have : "30000 free requests per month (1000 in 24 hours).". Using google looks fine too