Round favicon corners and soften icon background (#464)

Adds 16% rounded corners to the browser favicon set (icon.svg, favicon.ico, favicon-16x16, favicon-32x32) and switches the tile background from pure white to 90% white (#e6e6e6) so the icon edge stays visible on pure-white surfaces. apple-touch and android-chrome stay full-bleed squares since iOS and Android round them at the OS level.

Author: Sergio-Linkana