[lib] Migrate JoinThread/LeaveThread notifTexts to use EntityText

Summary: Depends on D6579

Test Plan: Generated JoinThread and LeaveThread messages on a web client in a chat with a different test user that is logged in on the iOS simulator. Background the iOS app and make sure the notif looks right

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6580