Make Segoe default font for Windows

Summary: Helvetica on modern Windows systems doesn't display very well, bolds are fairly not bold and leading between characters is all over the place. This change picks Segoe as the default Windows font which is easier to read, and the bolds are much crisper. There is even a slight improvement in text density with the improved leading.

Test Plan:
I've been using this locally for a few months, overall it's fairly similiar to Source Sans which people seem to enjoy.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: hach-que, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11825