Set up HD favicon for our sites.

[scottgonzalez]

@Krinkle:

Most if not all sites only have 16x16 icons, they look crap on high-dpi displays (e.g. MacBook Pro with Retina display).

I'd recommend we switch to making/serving multi-res ico files containing an array of 48x48, 32x32 and 16x16.

jquery/infrastructure#174

[Krinkle]

Bump. They're quite pixely for our growing number of HiDPI-enabled humans and non-humans. 😿