[web] render user avatars in chat thread composer

Summary:
render user avatas in chat thread composer

This diff won't be landed until avatars is given the green light to be released

Depends on D7205

Test Plan:
Please see the screenshots to see the changes I made

Before:

After:

Reviewers: atul, ashoat

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7206