Snackbar plus page Visbility API

It would be nice to use the page [visibility API](https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API) to control whether to show a snackbar or not. We can at least check this _before_ showing new snackbars, and continue the queue once visible again. Not sure if we can stall the timer based on page visibility on currently showing snackbars. Will research more as it is upgraded.