Regression in participant count in summary report

Issue is identical to CRM-20170

The patch to correct it was essentially reverted by this commit: https://github.com/civicrm/civicrm-core/commit/ec7990445b7d99c52bab6763ccaafa6f6ffc0c14 - CRM-21121

Which references a crash when 'only_full_group_by' is enabled.

However, fixing the crash means that we have incorrect reports again. Can we fix the report count whilst not breaking the query in 'only_full_group_by' mode?

Original patch: John K. , reviewed by Eileen McNaughton Brian Shaughnessy

Subsequent patch: @megaphone, reviewed by Eileen McNaughton Brian Shaughnessy

Steps to recreate and notes from original issue:

The 'event summary' report doesn't display the correct number of attendees for free events.

When someone registers through the front end, they appear on the report (because a contribution is recorded).

When you then record someone through the back end they do not appear, because there is no contribution record for the free event.

Steps to recreate:

• Create a free event with online registration
• Register through the front end
• Create another registration through the back end
• View the 'Event Income Summary' report
• Notice that only 1 participant is counted