[web] Move avatar-related components to avatars/

Summary:
This is mostly for personal convenience.

Could be argued that these are "general-purpose" components, but I'd rather they live in a separate avatars/ directory with the accompanying components/utils/contexts/etc etc.

Test Plan:
Dragged the files to avatars/ and had IDE auto-fix imports.

Scanned changes to make sure things seem correct.

Also opened web app and made sure user/thread avatars continue to appear as expected.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8187