Fix a bug on Calendar event day views for all day events in certain timezones

Summary:
Ref T11816. This logic was correct, we just did all the work and then mostly threw away the results. This worked correctly anyway in some timezones.

Instead, actually use $min_date and $max_date.

Test Plan: In "America/Toronto" with server in a more-western timezone, viewed a "Nov 11 - Nov 12" all-day event, saw those dates.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16867