[web] render user avatars in message reactions modal

Summary:
render user avatars in message reactions modal

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

Depends on D7195

Test Plan:
Please see the screenshot below to see the changes I made

Before:

After:

Reviewers: ashoat, atul

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7196