[lib] Migrate CreateSidebar notifText to use EntityText

Summary: Depends on D6578

Test Plan: Generated CreateSidebar message 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/D6579