[web] Display loadingSpinner over EditUserAvatar when change in progress

Summary:
Show a loading spinner over EditUserAvatar when a save is in progress. Pretty straightforward. In next diff I'll quickly "disable" the edit button as well.

Depends on D8260

Test Plan:
Looks as expected:

Reviewers: ashoat, ginsu, rohan, inka, kamil, michal

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8261