[web] introduce ThreadAvatar component

Summary:
After some discussion with @ashoat and @atul about how to fetch the ens avatars we decided to have two separate components to render avatars: UserAvatar and ThreadAvatar. This diff handles ThreadAvatar.

This diff is pretty much a copy of D7262

Depends on D7273

Test Plan:
I was able to render all three avatar types: Emoji, Image, and ENS:

Emoji:

Image (by hardcoding const avatarInfo = { type: 'image', uri: 'https://picsum.photos/200' }):

ENS (by hardcoding const avatarInfo = { type: 'ens' } and the same ensAvatarURI I tested in D7262):

Reviewers: ashoat, atul

Reviewed By: atul

Subscribers: tomek, ashoat, atul

Differential Revision: https://phab.comm.dev/D7274