`Restorer` pattern is unnecessarily complex

[efstajas]

I implemented the Restorer utility in order to save & restore step contents while navigating back and forth within a flow. Turns out a way, way easier way to do it is to just persist all of the flow's fields in its state (passed to Stepper as context), as I'm doing it on jason/funder-onboarding. Same effect, with no need for implementing Restorer separately. We should refactor all the existing flows that use Restorer to persist state like the new flows in jason/funder-onboarding do.