Refactor "User#unfollow into a service

Why are we doing this work

Same benefits as #375061. Also, it makes sense to turn User#unfollow into a service after User#follow was turned into a service

Should be similar to the work done here !135060 (closed)

Implementation plan

1. Create app/services/users/unfollow_service.rb
2. Move the logic from User#unfollow into this service
3. Delete User#unfollow
4. Replace usages of User#unfollow with Users::UnfollowService