R1:e35d72f48998

Build a basic calendar view

Summary:
This is a very small step toward building a Status and possibly an Oncall tool.
Build a calendar view which renders months.

Much of my hesitance to bang these tools out is that dealing with
dates/calendaring is basically horrible, so I'm trying to ease into it.

This calendar is locale-aware and all that jazz.

Test Plan:
- See:
https://secure.phabricator.com/file/view/PHID-FILE-c07a9c663a7d040d2529/
- Verified that months have the right number of days, today is…