Add the golang-image-ico package, which is an incredibly small package
to handle the ICO container format with PNG inside. Some profile photos
look quite pixelated when displayed at this size, but it's better than
nothing, and any Windows support is just a bonus anyway.
Updates #1708
Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>