Record progress in tutorials

## What I did
I successfully completed a few tutorials, then came back to the web site a few months later.

## What happened
I couldn't remember which tutorials I had finished, and which ones I stlll needed to work on.

## What could happen
It could display a list of tutorials on the tutorials intro page, and each one could have some status indicator: untried, in progress, solved, new since your last visit, changed since you solved it.

Use the browser's [local storage] to record progress.

[local storage]: https://developer.mozilla.org/en-US/docs/Web/API/Window/localStorage

Start at the tutorials intro page and read the linked browser localStorage documentation. Define how tutorial progress and statuses should be represented and updated, then verify that the page displays the statuses and preserves progress between visits.

Written by the indexing model from the issue text.