support for unlockable flags (#3479)

## Description:

Add support for purchasable/gated flags.

* Create a new "Store" modal that renders both skins & flags
* move all store related logic out of TerritoryPatternsModal
* use nation:code for existing nation flags & flag:key for gated flags
* check if user has the appropriate flags before purchasing

## 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

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

evan

src/client/graphics/layers/WinModal.ts

@@ -203,7 +203,7 @@ export class WinModal extends LitElement implements Layer {
               .requiresPurchase=${true}
               .onSelect=${(p: Pattern | null) => {}}
               .onPurchase=${(p: Pattern, colorPalette: ColorPalette | null) =>
-                handlePurchase(p, colorPalette)}
+                handlePurchase(p.product!, colorPalette?.name)}
             ></pattern-button>
           `,
         )}