Fix date-picker z-index to be above dialogs

Summary: Fixes T5880.

Test Plan:
  - Used event dialog in Calendar.
  - Reviewed z-index.css history for likely conflicts, didn't see anything suspicious.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5880

Differential Revision: https://secure.phabricator.com/D10263