Remove "viewerDateFrom" / "viewerDateTo" in favor of CalendarDateTime methods

Summary: Ref T10747. The CalendarDateTime object now carries the viewer timezone as part of its state, so we don't need to have separate accessors.

Test Plan:

• Viewed events, checked that crumbs render properly.
• Edited events.
• Created new events.
• Viewed calendar.
• Viewed event detail pages.
• Viewed profile mini-calendar.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16662