Standardize and streamline validation logic

[ghost]

Refactor validation logic across the solution to push validation into the Model classes so it's compartmentalized and allows the ViewModel to make a simple call (e.g. MyClass.Validate()) then take appropriate action based on the result. This approach is used in the Entities so the enhancement would largely be focused on bringing it to the ViewModels. A few options to consider would be the use of IValidatableObject, IValidationRule, and/or the use of ObservableValidator.