Summary:
Allow requests from untrusted origins instead of returning an error, but don't allow credentials (Cookie header) or API keys (Authorization header).
Allow setting the header `Content-type: application/json` as an alternative to `X-Requested-With: XMLHttpRequest` to make it easier for clients to make POST/PUT/PATCH/DELETE requests without authentication.
Discussion: https://grist.slack.com/archives/C0234CPPXPA/p1666355281535479
Test Plan: Added and updated DocApi tests. Tested manually how this affects requests made from a browser.
Reviewers: paulfitz, dsagal
Reviewed By: paulfitz, dsagal
Differential Revision: https://phab.getgrist.com/D3678