Use this decorator on Flask endpoints that should respond with CORS
headers. These headers are sent in a reply when the browser sends an
`Origin` request header; for more info see [1].
This commit rolls back the previous commit (0ee1d0d3), as this new
approach with a separate decorator is both easier to use and less
error-prone.
[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
