Don't mix different users on the same line in Calendar

Summary:
The current state is very confusing:
{F11734, size=full}

Test Plan: Display calendar for user with two different events in the same week.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2522