Fix CSS for textareas in windows

Summary:
Windows browsers default to monospace textareas, which look bad. Let's
use the same font we're using everywhere else.

Test Plan:
load differential in firefox on windows and see that the comment box in
differential is no longer Courier New.

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: btrahan, vrana, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3968