[CreatePhotosSlideshow] ffmpeg broken locally because of CORS related headers

[mkhatib]

The local server needs to serve our pages with 2 headers in order for the browser to allow us to use SharedArrayBuffer which ffmpeg depends on. This works non-locally because we already set these headers in our deployed server.

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp