build: Generate extra (marketing) icon sizes (#403)
build: Generate extra (marketing) icon sizes (#403)

The build script distinguishes between 'internal' (to be bundled with
the extension) and 'external' (not :-)) icons. The external icons are
simply placed in the PNG cache directory, so they can be picked up
manually, but may also contribute to speeding up builds for other
browsers.

Closes #380
matatk/landmarks,matatk/landmarks,matatk/landmarks