use a UNION instead of an OR for CalendarEvent#child_event_participants_scope
use a UNION instead of an OR for CalendarEvent#child_event_participants_scope

fixes CORE-2797

for large tables, this can increase performance from several seconds to a few
milliseconds, since it forces postgres to materialize the user id list first,
rather than scanning the entire users table.

Change-Id: Iaba16e9ad78b65c69031fdf3a4f50595c0a02c95
Reviewed-on: https://gerrit.instructure.com/190641
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Product-Review: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Tested-by: Jenkins

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms