[web] render user avatars in chat screen

Summary:
Rendered the user avatars in the chat screen. An important thing to note about this diff vs D7067 is that since web has a lot more screen real estate than native, I DID NOT reduce the width of chat bubbles. I thought about it at first; however, after seeing how the chat screen looked visually with the avatar, I felt that it was unnecessary to reduce the width of the chat bubbles as we did in native

Depends on D7194

Test Plan:
Please look at the screenshots below to see the changes I made (only handles the user avatars in the chat screen and none of the thread avatars shown in the UI--- those will come later in the stack)

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Under the hood:

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Responsiveness:

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Responsiveness with super thin photos:

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Reviewers: ashoat, atul

Reviewed By: ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7195