Give Calendar days from adjacent months a background color hint

Summary:
Ref T11326. When viewing "February", add a class to dates in January and March to let them be styled a little differently as a UI hint.

For now, I've given them a grey background. (Calendar.app changes the date number color instead.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16336