Serve CORS open headers for api.nasqueron.org JSON files

Summary:
As the API is meant to be consumed by other .nasqueron.org and external sites,
this change serves wide open headers to https://api.nasqueron.org/*.json.

An immediate benefit is to be able to get rid of the proxy_pass configuration
in www.nasqueron.org previously used to access sites.json without CORS issue,
a kludge introduced as the same time than commit cd026d254f2fe.

Test Plan:

• OPTIONS header returns well a 204 and not a 405
• CORS works with D1304

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1303