feat: config.json option to show empty rings

The `showEmptyRings` option in `config.ts` can now be set via
`config.json`. This enables display of headers for rings that contain no
items.

src/config.ts

@@ -3,6 +3,7 @@ import { Item } from "./model";
 export interface ConfigData {
   quadrants: { [key: string]: string };
   rings: string[];
+  showEmptyRings: boolean;
 }
 
 export const radarName =
@@ -12,8 +13,6 @@ export const radarNameShort = radarName;
 export const getItemPageNames = (items: Item[]) =>
   items.map((item) => `${item.quadrant}/${item.name}`);
 
-export const showEmptyRings = false;
-
 export function isMobileViewport() {
   // return false for server side rendering
   if (typeof window == "undefined") return false;
@@ -31,4 +30,4 @@ export function assetUrl(file: string) {
 
 export function translate(config: ConfigData, key: string) {
   return config.quadrants[key] || "-";
-}
+}