convert import styles to themepark

Introduces presets which avoid much of the previous configuration
duplication. The original import files are now thin wrappers around
the themepark themes.

lib-lua/flex-base.lua

@@ -4,6 +4,10 @@ local flex = require('themes/nominatim/init')
 function flex.load_topic(name, cfg)
     local topic_file = debug.getinfo(1, "S").source:sub(2):match("(.*/)") .. 'themes/nominatim/topics/'.. name .. '.lua'
 
+    if topic_file == nil then
+        error('Cannot find topic: ' .. name)
+    end
+
     loadfile(topic_file)(nil, flex, cfg or {})
 end