If the user specifies the `--no-web-resources-cdn` or `--local-web-sdk`, we should use the local version of CanvasKit. `flutter.js` now has a flag that can be specified in the build configuration that tells it to load locally instead.
Also, added a link to the relevant docs in the web template warnings.
This addresses https://github.com/flutter/flutter/issues/148713
Also fixes https://github.com/flutter/flutter/issues/145559