Add a CORS header to the mock-api

[moondrek]

Problem/Feature

The mock-api is a great tool for building against partner- or affiliate-specific endpoints. The absence of CORS headers causes browsers to reject AJAX requests to the mock-api by default.

Description of Changes:

• Add a Access-Control-Allow-Origin: * header, same as the helix endpoint

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)