use api_url in project /subscribe

To review:

(on client js) git checkout solve-cors-subscribe-infinite-loop
npm i && npm run dev
pwd (to get the location)
(on console) modify the package.json so that platformsh-client: "file:///<HERE THE OUTPUT OF THE PWD>"
npm i && npm run dev:prod
Go to the project overview page (i.e. https://console.platform.sh/psh-internal-dev/i7dymnob47qsi) and in the network development tools search for subscribe call, make sure is calling to the proxy instead of the region (with region I mean i.e. https://us-2.platform.sh/api/projects/i7dymnob47qsi/subscribe)

platformsh / platformsh-client-js