R1:3c4f31e4b921

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