Poll instance list

[david-crespo]

I ended up leaving this out of #2360 because I felt it was a little aggressive to always poll if any instance on the current page is in a transitional state. However, after you see auto-polling on the instance detail page, it feels like a big lack on the instance list, particularly when you start or stop an instance from that page. It changes to stopping and then sits there, which feels quite useless.

Because #2360 established a visual indicator that we are polling on instance detail (spinner next to the state badge), I feel more comfortable being clever in our logic around when to poll. Some ways it could work (not mutually exclusive):

• Poll for up to N seconds after an action if any instance in scope is transitioning
• Poll for up to N seconds after initial pageload if any instance in scope is transitioning
• Increase polling interval after a certain amount of time polling

See also #2376 for fancier polling logic on instance detail.