[web] Introduce barebones EditThreadAvatar component

Summary:
Introduce barebones EditThreadAvatar component. For now just shows spinner if threadAvatarSaveInProgress.

EditThreadAvatarMenu will be introduced in next diff.

Depends on D8801

Test Plan: Appears as expected in ThreadSettingsGeneralTab when editingAvatarsOnWebEnabled is true.

Reviewers: ashoat, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8802