Allow impersonation of customers

[wr-dev]

Describe the solution you'd like
It would be great if, as an admin, I could "log in as customer". We've used this feature a lot with NopCommerce, and it really helps to resolve customers' queries

Additional context
With Nop, you remain an admin with access to the administration app, but get to view the front end as the customer. As Umbraco separates members and users, this would need some kind of functionality to allow you to log in to the front end as an admin user, but in the context of a customer (so, authenticate the customer, using admin credentials)