Disable CORS management

[etchegom]

I built my django app with an nginx proxy upstream, which is in charge of handling CORS headers. I would need a new boolean settings value (like OIDC_CORS_MANAGEMENT_ENABLE) to tell django-oidc-provider not to manage CORS headers at all, so that I can delegate this job to my proxy.

Please tell me what you think about that. Maybe there is a particular reason for CORS headers to be handled like it is.
I forked the repo and made a new branch from develop, if that can help.

[juanifioren]

The spec says SHOULD support CORS so it's a good idea @etchegom