Rename BaseEditUserAvatarProvider to EditUserAvatarProvider

Summary:
Quick followup to D8347.

Just a simple rename + imports autofixed by IDE. Kept it as a separate diff from D8347 to make things easier to review.

Depends on D8347

Test Plan: Flow/ESLint/CI

Reviewers: ashoat, ginsu, rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8348