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

Author: evanpelle

index.html

@@ -204,11 +204,11 @@
           inline
           class="hidden w-full h-full page-content relative z-50"
         ></join-lobby-modal>
-        <territory-patterns-modal
+        <store-modal
           id="page-item-store"
           inline
           class="hidden w-full h-full page-content relative z-50"
-        ></territory-patterns-modal>
+        ></store-modal>
         <user-setting
           id="page-settings"
           inline