move icons to correct location, update usages in tooling

[shiftkey]

Resolves #559

Thanks to @sarim for identifying the root cause.

The bit that I'd forgotten as part of this is that static assets need to be defined in a platform-specific subdirectory to be included as part of the app package - app/static/win32 for Windows, app/static/darwin for macOS and app/static/linux for Linux.

This is the relevant code that does the actual copying:

desktop/script/build.ts

Lines 253 to 255 in c7c2cf6

	if (fs.existsSync(platformSpecific)) {
	fs.copySync(platformSpecific, destination)
	}

app/static/logos is only used at packaging time, which is why things were working sometimes.

I also found out that the icon option for electron-packager is not supported on Linux, so I've updated that area to be clearer.