Cleaning up day view clusters for events shorter than about 30 minutes

Summary: Cleaning up day view clusters for events shorter than about 30 minutes

Test Plan: Create two event within 30 minutes, 1 minute long, each. Day view should properly cluster the events.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12799