[keyserver] Render raw chat mention in the notification text

Summary:
Solution for ENG-4562.

This diff enables rendering raw chat mentions in notification text.

Depends on D9005.

Test Plan: Follow the test plan from D9007.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9218