feat(routesCdnPath): Allow users to specify a custom local nav file

Closes #581

This provides a `.configURL` config parameter on `routesCdnPathProvider`, so you can run a local nav JSON file during development. 

This will be the preferred way for people to test changes to the official encore-ui-nav files. Do local changes using this, then send a PR against encore-ui-nav with the modified JSON.
Could in the future we benefit from being able to add "App" warning messages? or expose a way to do it?
Granted I guess that could be within the *rx-page* rather than outside *rx-app*, and we only reserve these for very special cases (preprod/local nav). 

I just see this and immediately think "we ought to refactor this and not hardcode it".  But these are very special cases I guess