"M" icon looks a little low-res on the new-tab/home page

I'm not sure which asset was used, but it looks a little blurry on my monitor:

We should use a 2x or 3x asset instead to improve its sharpness on high-res displays. This fix could potentially wait for @ruihildt's final decision on whether or not we should go with the tiled or non-tiled version first though :)