If no path is supplied, browsers will look for the map on the same path as the JS file itself. This fixes two problems that we see in production:
1. When compiling assets against one CDN, and then re-using them on a site with a different CDN, the sourceMappingUrls would be incorrect and print warnings in the console
2. If both an S3 CDN and an app CDN are configured, we were using the S3 CDN for the JS and the app CDN for the map. This commit will make sure we use the S3 CDN for both.
fcae4a8faf