Warn before reloading if unsaved cells

[jrus]

Also see my previous post about this topic,

bug: page navigates away with unsaved changes in a text box

While typing a long function in a cell, I somehow got my copy of Firefox here to interpret the backspace key as page navigation before my typing had been committed. The browser navigated back in history, leaving the page. When I went forward again to return to my notebook, my typing was lost. This reminded me that I need to disable the moronic default backspace behavior in my Firefox here, but there is also an Observable bug here (which I have run into a couple of other times for other reasons,…