[web] render thread avatars in thread picker modal

Summary:
render thread avatars in the calendar thread picker modal

Depends on D7275

Test Plan:
Please view the screenshots 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/D7276