Further simplify the AuthenticationViewModel and the public interface of AuthenticationServiceProvider.
Publish currentActiveUser from AuthenticationServiceProvider instead of assuming all callers know to take the first in the list as the active one.
contributes to iOS-319
Also, AppContext, APIService, and AuthenticationServiceProvider are now more obviously singletons.
And AuthenticationServiceProvider can now be asked for the current active user, instead of every caller assuming the first element of a list of users is the active user.
