LTN red heatmap scale

[dabreegster]

When you filter a road and just push traffic somewhere else, the red "moves over" quite intuitively/nicely. But when you create a cell and alter the total number of rat-runs, everything re-scales. In other words, there's always some street that has the most rat-runs, so there's always a dark red street. This might be expected. If you start out with some street having a max of 50 rat-runs, and after some changes, the worst street only has 3, maybe we should keep using 50 as the max, and color that street with only 3 as very light red.

CC @dingaaling

[dabreegster]

Dupe of a-b-street/ltn#74