Find a better way to deal with lack of resource manifest in DEV

[DanielBaulig]

Currently we simply don't load the resource manifest and don't try to cache the app resources. This makes development easier and simpler, but also means that the code path is only tested if explicitly built and run in preview mode.

npm run build && npm run preview

And any issues that need to be debugged within that setup have to be debugged in minified code, which makes it a real headache. I need to find a way how I can reasonably run the resource manifest / app resource cache code during development, too. Maybe point the resource manifest at a dummy manifest that contains some dummy app resources.