Avoid an n+1 query problem and fetch the bundled aggregations for
m.annotation 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) and threads
in b65acead428653b988351ae8d7b22127a22039cd (#11752).