Avoid an n+1 query problem and fetch the bundled aggregations for
m.reference relations in a single query instead of a query per event.
This applies similar logic for as was previously done for edits in
8b309adb436c162510ed1402f33b8741d71fc058 (#11660; threads
in b65acead428653b988351ae8d7b22127a22039cd (#11752); and
annotations in 1799a54a545618782840a60950ef4b64da9ee24d (#14491).