Dynamically composite favicons from customizable sources
Summary: Ref T13103. Make favicons customizable, and perform dynamic compositing to add marker to indicate things like "unread messages".
Test Plan: Viewed favicons in Safari, Firefox and Chrome. With unread messages, saw pink dot composited into icon.
Maniphest Tasks: T13103
Differential Revision: https://secure.phabricator.com/D19209