There seems to be a front-end library that can do the clustering. `leaflet.markercluster.js`, https://github.com/Leaflet/Leaflet.markercluster
 Should work for the nearby feature or for user added feature collections on maps.
 Nearby and normal markers should be done separately, the code is different.
Documentation of design decisions:
- Numbered markers and grouped-count markers are styled differently to make them distinguishable from one another.
- The pin groups will always have the same color, regardless of the colors of grouped pins within.
- Clustering will not be an optional setting (e.g. for readers or per map).
Open design questions:
- When we’re highlighting a specific marker, should that be kept out of the clustering? (and styled in some special way?)
- What happens when clicking on a clustered marker? Do the individual pins appear? Does the map automatically zoom in?