Add platform detection and a Windows-specific monospaced font override

Summary: Use UA strings to detect platform; override general monospaced settings with platform-specific ones. Fixes T2868.

Test Plan: whatcouldgowrong

Reviewers: chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2868

Differential Revision: https://secure.phabricator.com/D5526