Add HiDPI favicons

Summary: Fixes T5939. Specifically, 16, 32, and 64 bit pngs are in the .ico file. Apple icons are added to support non-Retina and Retina iPad and iPhone. Follows roughly Apple guidelines here: https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html

Test Plan:
Tested Chrome, Firefox on Mac. Much cleaner on Retina.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5939

Differential Revision: https://secure.phabricator.com/D10796