allow reuse of structure discounts (#1513)

## Description:

When someone loses their structures they get their bonus back. Also keep
the structures built, it's nice to still get the bonus after capturing
someone's city.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I have read and accepted the CLA aggreement (only required once).

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/core/game/Game.ts

@@ -7,7 +7,6 @@ import {
   PlayerUpdate,
   UnitUpdate,
 } from "./GameUpdates";
-import { PlayerView } from "./GameView";
 import { RailNetwork } from "./RailNetwork";
 import { Stats } from "./Stats";
 
@@ -131,7 +130,7 @@ export enum GameMode {
 }
 
 export interface UnitInfo {
-  cost: (player: Player | PlayerView) => Gold;
+  cost: (player: Player) => Gold;
   // Determines if its owner changes when its tile is conquered.
   territoryBound: boolean;
   maxHealth?: number;