Add allday, multiday logic to AphrontEventView

Summary: Hardens up the logic for DST and makes them easier to access elsewhere.

Test Plan: view sample events, all day and multiday, in my sandbox

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8332