QOL - swapping years - no indicator to show loading

[celinaolivieri]

Describe the bug
When I swap between years on CESI, I do not get a visual indicator to show the map is loading data.

To Reproduce

1. Open CESI
2. Load two data sets (Carbon Monoxide and Average fine particulate matter)
3. Click Historical
4. Click 2015. Wait.
5. Click 2018. Wait.

Expected behavior
I'd like to have a visual indicator in the legend panel that the layer is loading.
Actual behavior
The layer does not show up until it is loaded. This was confusing to me because it took 10 seconds to load, and I thought my layer disappeared.

Environment
Edge