Regenerate editingStatsId on unload
So that the ID isn't reused when the user uses the back button
to navigate back to an edit page.
It would be more natural to use pageshow for this, but that
event also fires on initial load. There is a .persisted property
that indicates whether the page was recycled, but that
property doesn't work in Chrome:
Doing this from pagehide doesn't work either, because that
runs before unload (at least in Chrome) and causes the
abort event to be emitted with the new ID. So instead,
regenerate the ID in the unload handler after the
abort event has been sent.