[web] introduce UserAvatar component

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

This is also pretty much a copy from D7246

Depends on D7192

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 with in D7246):

Reviewers: ashoat, atul

Reviewed By: atul

Subscribers: tomek, ashoat, atul

Differential Revision: https://phab.comm.dev/D7273