Add multiple base layer options

The current base layer for Observe is Mapbox Streets. The styling on this makes it difficult to distinguish between editable and non-editable features.

While work is underway to enable additional editable features, there are multiple use cases where less distraction would be preferred. As a future enhancement, consider replacing Streets with a different base layer or specifying this in the Config. The Mapbox Light base layer can help filter out non-esseential features. In other cases, the contours of Mapbox Outdoors may be preferred to the hillshading of these other layers.

As a further enhancement, consider enabling multiple base layers for selection by users in the layer toggle.