Offer OpenStreetMap as an alternative tiles layer to MapBox
https://www.openstreetmap.org/copyright tells me the tiles are CC BY-SA 2.0.
I would like to use openstreetmap instead of MapBox, not only as a way to save the GNOME Foundation some money, but because OpenStreetMap tiles are more accurate and up-to-date. Mapbox's tiles regularly frustrate me because they're months out-of-date compared to the state of OSM.