Display holidays

Summary:
We will need it for two purposes:

• Status tool.
• Nagging tool - @aran suggested using "3 business days" and I don't want it to fall on New Year's Eve or such.

I don't plan working on any interface for editing this as this kind of data should be always imported.

Test Plan:
bin/storage upgrade
scripts/calendar/import_us_holidays.php
/calendar/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2375