Update how GADM basemaps tables are handled

[edwardchalstrey1]

• [x] Update the populate_gadm script so it runs the last SQL steps currently in spatialdb docs
  • [x] Could also make the printout a bit more informative
• [x] In GADM, set the colour of "Modern Caspian Sea" to blue and don't add "Modern". Do this for all seas.
• [x] Try the GADM basemap with a clear fill instead of white