[web] Add showSpinner prop to ThreadAvatar

Summary:
We need this for when we introduce EditThreadAvatar.

We just add showSpinner prop to ThreadAvatar to be passed to underlying Avatar component.

Depends on D8800 (kind of)

Test Plan: Tested implicitly by subsequent diffs (observed that spinner spun when thread avatar update was in progress).

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8801