Make Calendar month view events render a little more consistently

Summary:
Ref T11326. This just inches things forward a little bit:

• Make it easier to see current day.
• Line-through cancelled events.
• Don't colorize the whole event title, just use an Attending/Invited/Custom icon.
• Slightly subtler treatment for all-day events.

Test Plan: See screenshot in T11326.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16306