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
