Update ExternalServicesStore.List to allow filtering by user

Created by: unknwon

..., and site admins are able to list all.

Refs:

• https://sourcegraph.com/github.com/sourcegraph/sourcegraph@5d73f94a7ad9c8c5a5d30954f737e9dde5c2a8b5/-/blob/internal/db/external_services.go#L405

Part of #12699 (closed).