[keyserver] Drill MessageDatas into notificationCollapseKey

Summary: Depends on D6933

Test Plan:
Flow, and tested in combination with the next diffs to confirm that getting @-tagged in a message that triggers the creation of a sidebar will update the SIDEBAR_CREATION notif
to be suffixed with "and tagged you"

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6934