MDL-58768 calendar: Add $requestinguser to the container class
The requesting user is not always the same as the current user. The container class now allows a different user to be set as the requesting user. All capability checks are done against the requesting user. Also, the $requestinguser is passed to core_calendar_provide_event_action and core_calendar_is_event_visible callback functions. These callback functions need to be updated in all activity modules to accept a $user parameter.