GitLab

POST for application/json are safe from csrf but not multipart/form-data.
CSRF protection is thus disabled on application/json (no matter the authentications method).

For multipart/form-data, there are 3 usecases:

1. multipart/form-data authenticated by cookies (webrowser), this requires
   csrf and this is handled by MultipartRqlIOController.
2. multipart/form-data anon user, this does not require csrf and this in
   handled by AnonMultipartRqlIOController
3. multipart/form-data authenticated with authorization, this does not
   requires csrf as there is an authentification. This is not handled here
   but in signed-request that implements the authentification.

Lets us extend it later without breaking clients.

Related to #3639007.