add basemap to the thumbnail

[boney-bun]

the current thumbnails are a result of only the rendered layers.
It would be nice if the thumbnails also draw the basemap on the layers' background. so the user can easily see where the layers fit on the map.

child of #457

[gubuntu]

• this is already implemented in some GeoNode instances (such as haitidata.org) so hopefully we can do the same
• map thumbnails alreday flatten multiple layers into the thumbnail png so it's just a matter of adding a basemap response to the flattening process (or are map layers returned in one already-flattened response by QGIS?)