Major cities missing on low zoom levels

At the moment [only (state) capitals](https://github.com/pnorman/spirit/blob/cb281589fbeeb759ecf3a97cf8f4eeeff56236cb/themes/shortbread/topics/places.lua#L75-L85) getting rendered on low zoom levels. Though those are not always the mayor city you expect to see in the map. Eg. [New York, Chicago, Detroit and Milwaukee](https://www.openstreetmap.org/#map=7/42.078/-82.227&layers=S) are not shown on that level. Similar in [Germany](https://www.openstreetmap.org/#map=7/50.719/8.731&layers=S), where Wiesbaden is shown, though the larger and more known Frankfurt is missing, same for Düsseldorf and Colongne/Köln It might be useful to add as well cities with large population to that zoom.