Fix CORS policy on source manager stage #600
Description
Got an error when trying to submit the label/annotate page in pdap.dev. Let's fix so we can test properly!
Access to XMLHttpRequest at 'https://source-collector-app-stage-eqjje.ondigitalocean.app/annotate/anonymous/3509?session_id=4e1a3362-2f86-4c12-b35c-35df7c5d60b9' from origin 'https://pdap.dev' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource.