F map marker color config

Summary

Minimal enhancement to make the marker colors configurable (for the entities on the map) as requested per caosdb/customers/leibniz-zmt/management#127

Focus

Leaflet has a particular data structure (DivIcon_options) for the configuration of the map markers. We just use that and read it out from the config.

Test Environment

Use a normal map.json configuration and merge it with

{
    "entityLayers": {
        "current_page_entities": {
            "icon_options": {
                "html": "<i class=\"bi-geo-alt-fill\"  style=\"font-size: 20px; color: teal;\"></i>"
            }
        },
        "all_map_entities": {
            "icon_options": {
                "html": "<i class=\"bi-geo-alt-fill\"  style=\"font-size: 20px; color: brown;\"></i>"
            }
        }
    }
}  

You should now see the icons in brown and teal, instead of red and blue.

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