[User Model] Refactor model/op repo/login to be inline with expected scenarios

[brismithers]

• Rework modeling framework to support nested models
• Move identity and property models to be singleton model stores
• Tune/comment operation repo concept
• Work through login/logout process with new operation repo/model structure
• Remove IUserSwitcher as it is no longer needed due to updates to model framework. UserManager now reacts to changes to the appropriate model stores.
• Build out to-be backend services.

---

This change is