[lib] Reactions only bump thread timestamps when reacting to viewer

Summary:
This addresses ENG-9558 for reaction robotext messages. The implementation is broadly similar to D13828.

Depends on D13829

Test Plan:
I logged in as two users on web, and with the second user, tested reacting to the first user's messages and other messages. Confirmed that the timestamp was bumped for the first user only when reacting to their messages. I also tested this with a sleep to confirm that the timestamps would only be bumped after the promise resolved

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13918