Reset the user's cached sticker packs when they (de)select packs

Otherwise we'll display stale packs and the user won't see updates. Part of #156

src/api/dimension/DimensionStickerService.ts

@@ -114,6 +114,8 @@ export class DimensionStickerService {
 
         userPack.isSelected = request.isSelected;
         await userPack.save();
+        Cache.for(CACHE_STICKERS).del("packs_" + userId);
+
         return {}; // 200 OK
     }