Prefer ephemeral sessions in ASWebAuthneticationSession

Tusker/Screens/Onboarding/OnboardingViewController.swift

@@ -82,6 +82,8 @@ extension OnboardingViewController: InstanceSelectorTableViewControllerDelegate
                 }
             }
             DispatchQueue.main.async {
+                // Prefer ephemeral sessions to make it easier to sign into multiple accounts on the same instance.
+                self.authenticationSession!.prefersEphemeralWebBrowserSession = true
                 self.authenticationSession!.presentationContextProvider = self
                 self.authenticationSession!.start()
             }