Admin neighborhood completion histogram should have variable height

[misaugstad]

Brief description of problem/feature

The "Coverage Percentage by Neighborhood" graph on the Admin page looks nice for some cities (like Seattle). Anything with around 50 open neighborhoods looks decent. But we have a fixed size for the chart. If there are only a couple of cities, or if there are many cities, it looks pretty bad (or is unreadable).

Expected behavior

This graph should change height depending on the number of neighborhoods that we are trying to show on the chart.

Across our deployments, we have anywhere from 1 to 1,049 neighborhoods in a given city. It would be nice for this chart to be legible and nice looking for any such deployment. I could see there being some sort of a cap; I'm not sure that we can show 1k neighborhoods in this sort of format easily. But we should see how close we can get without having to do a big overhaul!

It should be easy enough to actually set the height dynamically; we're choosing the height in JavaScript in a place where we already know the number of neighborhoods. It's just about playing around with a formula that will look nice across a wide range of deployments.

@nschung28 I'll have you take this on for now! You can create synthetic data in JS to make it easy to test the look for varying numbers of neighborhoods.