Implement day view event clustering so that overlapping events can share latitude

Summary: Ref T4393, Implement day view event clustering so that overlapping events can share latitude.

Test Plan: Open day view in Calendar search query on a day with overlapping events. Overlapping events should be displayed side by side when possible, and should be layed out as a staircase when overlapping times.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T4393

Differential Revision: https://secure.phabricator.com/D12711