Right now we only set the ip and last_seen_at fields when a customer record is created. Let's update the customer update_metadata endpoint (https://github.com/papercups-io/papercups/blob/master/lib/chat_api_web/controllers/customer_controller.ex#L58) so that it also handles this.

(I think we can just default the last_seen_at field to the current timestamp for now.)

Description:
Implement MVVM(Model View ViewModel) architecture in the project.