[web] Add showSpinner prop to Avatar component

Summary:
Add showSpinner?: boolean prop to the Avatar component and display LoadingIndicator over Avatar if set to true.

Depends on D8283

Test Plan:
Looks as expected:

Reviewers: ashoat, ginsu, rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8285