Add UI/UX treatment to indicate layers or stories are "loading" when I click to play them

[jonpmarino]

When a user clicks on a layer or story to view from the Explore page, it often takes a few seconds for the layer or story to fully load the on-map content. Without a UX treatment, it appears to the user that something is broken or wrong. The user should not be able to play a layer or story until it is fully loaded and ready to play seamlessly.

Perhaps add a loading circle on the map, or a buffering progress line somewhere. On RETRO, we had a buffering progress line at the bottom of the map canvas.