implement localStorage solution to maintain state between sessions

[hamilton]

Given that this app uses react + redux, and the stored state of the app is fundamentally just a big json object, we can save this in local storage so that this early-stage app can persist for a particular user.

As a starting point, let's simply make a big 'save' button to save the state to local storage. I attempted to save the state to local storage with each state change (which happens with every cell edit right now) and that made the app a little fussy.

We can use #16 to capture the name of the notebook, which can be saved w/ a simple, serialized key-value pair.