Use a modern query object to query Calendar events

Summary: Ref T4375. Calendar uses oldschool `loadOneWhere()` calls. Make CalendarEvent policy-aware, do the edit/delete policy checks through the policy framework, and use modern query infrastructure.

Test Plan:
  - Viewed calendar;
  - created, edited, deleted event;
  - viewed calendar tab in Conpherence.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4375

Differential Revision: https://secure.phabricator.com/D8146