Skip CSRF checks if no cookies or if authorization: bearer xxx headers

[simonw]

Needed by Datasette in simonw/datasette#835

If an incoming request has no cookies there's no point in CSRF protecting it... UNLESS it's to a login form to protect againts login CSRF attacks. So the middleware should have an option for "always CSRF protect these paths" to allow /login to be protected.

If an incoming request has a Authorization: Bearer xxx token there's no need to CSRF protect it because regular user requests from authenticated browsers can't include the Bearer prefix - they will always look like this instead (which should be CSRF protected):

Authorization: Basic QWxhZGRpbjpPcGVuU2VzYW1l