Added the resource economy, its works and their icons

- Five resources (energy, steel, food, luxury, high-tech) with per-city stores, a world market, delivery and material upkeep.
- Resource-producing tile works built outside cities; every nation opens with the works its economy needs.
- Real game-icons.net art replacing the dummy icons for the new resources and works, with the credits updated.

server/game_server.js

@@ -21,6 +21,7 @@ const DELTA_COLLECTIONS = [
   "tileEthnicity",
   "cityStats",
   "visible",
+  "resourceGraph",
 ];
 
 // A wire move order carrying more units than this is split into chunks that
@@ -216,6 +217,8 @@ export class GameServer extends EventEmitter {
         return this._handlePolicy(civ, order);
       case "revoke_policy":
         return this._handleRevokePolicy(civ, order);
+      case "set_interest_rate":
+        return this.state.requestSetInterestRate(civ, Number(order.rate));
       default:
         return false;
     }