Add functionality to customize the z-index of the layers present within the map

Currently, the z-index of the layers/overlays of our map are fixed. A number of users wanted to be able to reorder the layers. For example, rendering a raster layer over the countries layer can make it difficult to discern how a country is affected, as the country borders lie below the raster. The ability to move the layer upward would circumvent the issue.

Adding the feature to the map may be best split into two stories and should be discussed prior to start - this is because the overlays are rendered independently; whereas, the base map layer consists of a number of layers. This means the logic employed will likely be dissimilar, or the base map will simply require a change of approach altogether and so that refactor should be done independently

Start by locating the map rendering entry points for independently rendered overlays and the multi-layer base map. Discuss whether to split the work before implementation, since the two layer types may need different approaches or a base-map refactor. Done means users can reorder layers or customize their z-index without obscuring important map features.