Consider adding other contextual layers to HTML preview maps for alerts

[rudokemper]

For some users, there may be existing geospatial layers that can be helpful to add to the HTML preview maps, to add additional context about the nature and severity of the alert in addition to a satellite imagery background map and the t0/t1 scenes used to detect the alert. For example, for small-scale gold mining, having a historical layer of gold mining activity can help the user understand if a new alert is an addition to an existing mining area, or a new one. Let's figure out a way to add this kind of contextual data for each territory_id.