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: {F1738951}
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11326
Differential Revision: https://secure.phabricator.com/D16335