fix: on React, load marker images before adding layers

[rudokemper]

Resolves #965.

In the Map component, this PR refactors the this.map.on("load") event to prioritize loading the marker images before adding the Terrastories point data and layers. This approach addresses a race condition where the layers might sometimes be added to the map before the marker images had fully loaded.

To improve code clarity, I've also extracted the layer addition logic into its own function, addPlaceMarkerLayers().

Before:

After:

In so doing, the following warnings in the console have also gone away:

mapbox-gl.js:31 Image "ts-marker" could not be loaded. Please make sure you have added the image with map.addImage() or a "sprite" property in your style. You can provide missing images by listening for the "styleimagemissing" map event.
mapbox-gl.js:31 Image "ts-marker-cluster" could not be loaded. Please make sure you have added the image with map.addImage() or a "sprite" property in your style. You can provide missing images by listening for the "styleimagemissing" map event.

[rudokemper]

@lauramosher I know our goal is to eventually switch over to the Explore UI which does not have this issue, but hoping this could work as a temporary resolution. This race condition issue shows up every time for some of our online users and it's not great when they're doing a demo.