[white-labelling] The build system needs 64px and 16px icons

[davesag]

The build system assumes the presence of 64px and 16px icons, though they're not required by the browsers. The build system puts references to each of those in the generated manifest.json, but if they are not present the browser throws an error.

1. Browsers don't use the 64px rendition at all

2. The build system should either

   (a) check if the lower-res renditions exist on the assets path or
   (b) add a setting to build-config to list which renditions should be referenced in the generated manifest.json.