Modernize DatePicker

Summary: Fixes T5893. Uses more standard colors / spacing / shadows / icons.

Test Plan:
pick dates

{F191310}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5893

Differential Revision: https://secure.phabricator.com/D10285