[web] Convert RobotextMessage to hook

Summary:
This will make it a little easier to call useENSNames, and the component is better suited as a hook anyways.

Depends on D6526

Test Plan: Make sure that robotext messages still render on web. Also Flow

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6527