Map marker shadows render weirdly when many markers close together

[rjmackay]

I think part of the problem here is that we're not managing shadows separately, as leaflet does with its built in markers. I'm not 100% what the right way to do this is but its likely we need to extend L.Icon and render an additional HTML element for the shadow. This is then added to the leaflet ShadowPane, which site below the marker pane.. ensuring shadows don't overlap other markers.

We might be able to use https://github.com/hiasinho/Leaflet.vector-markers to handle much of this.

[rjmackay]

I made an attempt to fix this https://github.com/ushahidi/platform-pattern-library/tree/map-shadows

However I can't figure out the CSS-fu to generate a sensible shadow since I don't have the SVG present in the shadow layer