Display historical norm for data shown on map layer

[jcahail]

Since we have all the historical data (or should in most cases), it would be super helpful to show the historical norm to the user when they're viewing one of the map layers. The number of extreme precipitation days will be very different in Oahu vs. Texas. Since we're already using 1990-2000 as the baseline for the Top Hazards, perhaps we could do the same here.