Create strategy and documentation for changes for localStorage data

[ckarlof]

Going forward, I'd also like us to be a little more disciplined with how we handle data in localStorage. Bugs involving "legacy" localStorage state are very hard to test and reproduce. Here's what I'd like to see:

• Document our localStorage layout. This could be in an .md document in this repo.
• Version the localStorage layout with the train number. If a train changes the localStorage data in any way, we note it in the documentation and annotate the change with the train number. We can explicitly version data in localStorage or do it a more feature detection based way.
• Run upgrade code on each appStart to make sure everything is up to date. It might start to look something like this:

upgradeTo25();
upgrade25To27();
upgrade27To29();

[ckarlof]

Let's remember to do this once we make changes to the localStorage format.