Fix paths in manifest, extract base path in install script to avoid MANIFEST_URL_ERROR

This PR makes sure the manifest URL is valid by rebasing the location.href to the last directory. In fact, if the user launches the app on `/mdn-app-template/index.html` instead of `/mdn-app-template/`, the app fails to install with an obscure `MANIFEST_URL_ERROR`. I also fixed paths in manifest to match the repo name.