Create a global zindex object for layer management

[rguico]

We need to manage which layers are on top of other ones better, instead of defaulting to "whichever one we checked last".

The best way to do this is probably a global variable, something like the following...

var ZINDEX = Object.freeze({
    'densityBase': 1,
    'densityHighlighted': 2,
    'bikeLanes': 3
});

// set the layer zIndex
layer.setZIndex(ZINDEX.densityBase);

[stevevance]

There's a Leaflet method to do this, I think. autoZIndex

I believe that when it's set to true, which is the default, then the layers maintain their zIndex.