Revert "[lib] Disable chat mentioning"

Summary: This reverts commit 2a40ae7bdba712539348e23ba07d836196085de6.

Test Plan: Check if the chat mentions are being displayed.

Reviewers: kamil, inka, atul

Reviewed By: atul

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9465