Remove extra margins on Calendar month view

Summary:
Ref T11326. This doesn't go quite as far as the mock in T11326#185932, but gets rid of the easy margins.

Also cleans up some of the border rules so they're simpler and more consistent (no weird ragged edges on the far right).

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16335