[web] Introduce placeholder EditUserAvatar component

Summary:
Introduce placeholder EditUserAvatar component and include in AccountSettings if editingAvatarsOnWebEnabled is true.

This diff is super minor and EditUserAvatar is effectively an alias for UserAvatar.

I'm building this component incrementally in this stack and I think it made sense to separate this from subsequent diffs despite how trivial this one is.

Depends on D8221

Test Plan: Made sure that EditUserAvatar component appeared as expected after manually setting editingAvatarsOnWebEnabled to true.

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8222