Add date format preference and respect it in date selection controls

Summary: Ref T8362, Add date format preference and respect it in date selection controls

Test Plan: Set date format preference in the user settings panels, create new event, select new start date in the correct format.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: jasonrumney, eadler, epriestley, Korvin

Maniphest Tasks: T8362

Differential Revision: https://secure.phabricator.com/D13262