refreshGuestToken() runs indefinitely (and stacks!)

[gamma613]

In our react application, I've observed that superset embeds will continue running refreshGuestToken() after I've left the page that has the embed.

What's more, if I re-visit a page embedding a dashboard multiple times, it has a stacking effect - ie: if I visit the dashboard 6 times, it will now make 6 calls to refreshGuestToken().

I've tried calling dashboard.unmount() when my page unloads - which successfully removes the iframe, but it doesn't cause the setTimeout for refreshGuestToken() to be cancelled.

Not sure what other things could be floating around in memory as well.

[Vitor-Avila]

Addressed here #22