Show warning if the user tries to close the page without finishing

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One issue with interactive experiments is that the user might leave before finishing the experiment. We could catch the event and show a warning. If the user insists on leaving, at least we can save the intermediate results before they close the page.

See:

• https://stackoverflow.com/questions/13443503/run-javascript-code-on-window-close-or-page-refresh
• https://stackoverflow.com/questions/1631959/how-to-capture-the-browser-window-close-event