recommend against using `unload` event

[fergald]

Currently this article recommends using the unload event to close popups and send stats. unload cannot be relied upon and best practice is to avoid it entirely, using pagehide or visibilitychange.

E.g. popups should be closed in pagehide, not unload because a page that canenter BFCache will not fire unload but will fire pagehide

Stats should not be sent from unload because if the page goes into BFCache and the user never returns, the unload event will never fire.