fix(signin): cache credentials for desktop sign-ins, otherwise only cach...

...e email

fixes #1621

@ckarlof r?
Yeah, the relier could even have a method like `shouldNavigateAfterSignIn`. The problem here is that we need a way to signal to the functional tests that sign-in was successful, which is usually accomplished by checking that the next screen in the flow is displayed. But in a desktop context the UI doesn't change, so we'll have to get creative.