Give the Calendar month view a nice hover effect

Summary:
Ref T11326. Currently, we link Calendar days using hidden DOM nodes.

This is nice because it's simple, and right-clicking a day works properly. However, it's a bit ugly/unintuitive, messy, and unclear. It's especially messy because days are really two different rows, one for events and one for day/week numbers.

Instead, use JS to highlight day cells. You can still right-click by clicking the actual day number, which seems like a reasonable compromise.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16334