Check CSRF Referer against CSRF_TRUSTED_ORIGINS

[Joshua Kehn]

Django's CSRF implementation (when used over HTTPS) validates that the Referer header matches the current host (i.e. that the request is coming from the same domain).

There are cases where it is reasonable to allow non-same-origin CSRF-protected requests. Ticket #24996 (which also contains some discussion relevant to this ticket) has a pull request to allow any host that matches CSRF_COOKIE_DOMAIN past the Referer check; this takes care of the common subdomain cases.

But there also cases where a totally separate domain may be allowed to make POST (etc) requests to an API via CORS, and CORS headers can be configured to allow XHR requests from that external domain to send cookies to the API (including the CSRF cookie). But at the moment such a technique falls afoul of the Referer checking, and the only way to bypass it is by monkeypatching the request headers (that's what ​https://github.com/ottoyiu/django-cors-headers does).

This ticket proposes adding a CSRF_TRUSTED_ORIGINS setting, which can be set to a list of hosts that should be considered valid Referers for the purposes of CSRF checking.

[Carl Meyer]

I'm updating the description here to try to more clearly describe the motivation for _this_ feature, why it is useful in addition to #24496. Please let me know if you think the new description is lacking.