Reuse common getUser() / setUser() in PHUICalendarListView

Summary:
These methods already exist in AphrontView. The redefinition can sometimes cause this warning:

  [2014-02-24 18:27:48] ERROR 2048: Declaration of PHUICalendarListView::setUser() should be compatible with AphrontView::setUser(PhabricatorUser $user) at [/INSECURE/devtools/phabricator/src/view/phui/calendar/PHUICalendarListView.php:138]

Test Plan: Viewed calendar on profile.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D8329