Рефакторинг, очистка, работа над ошибками, связанными с базой, отказ от глобальной переменной $user во многих файлах.

Singleton в некоторых местах вместо решения #42.
Новые шаги для решения #16 и #52.
Closes #42.
Closes #32.
Closes #31.

shop.php

@@ -8,25 +8,25 @@ use Battles\Template;
 use Battles\User;
 
 require_once 'functions.php';
-Shop::$current = new Shop(Shop::GENERAL_SHOP);
+
 if (!empty($_GET['otdel'])) {
     if ($_GET['otdel'] === 'sale') {
-        Shop::$current->categoryType = Shop::CATEGORY_SALE_ITEMS;
+        Shop::id(Shop::GENERAL_SHOP)->categoryType = Shop::CATEGORY_SALE_ITEMS;
     }
     elseif ($_GET['otdel'] === 'market') {
-        Shop::$current->categoryType = Shop::BARTER_SHOP;
+        Shop::id(Shop::GENERAL_SHOP)->categoryType = Shop::BARTER_SHOP;
     }
     else {
-        Shop::$current->categoryType = $_GET['otdel'];
+        Shop::id(Shop::GENERAL_SHOP)->categoryType = $_GET['otdel'];
     }
 }
 
 if (!empty($_POST['sellshop'])) {
-    ShopItem::sellItem($_POST['itemId'], User::$current, 1);
+    ShopItem::sellItem($_POST['itemId'], User::getInstance(), 1);
 }
 
 if (!empty($_POST['buyshop'])) {
-    ShopItem::buyItem($_POST['itemId'], User::$current);
+    ShopItem::buyItem($_POST['itemId'], User::getInstance());
 }
 
 if (!empty($_POST['buymarket'])) {
@@ -42,20 +42,20 @@ Template::header('Магазин');
 <div class="status"><?= ShopItem::$status ?></div>
 <div class="row shop">
     <div class="left column">
-        <h3><?= Shop::$current->getCategoryName() ?></h3>
+        <h3><?= Shop::id(Shop::GENERAL_SHOP)->getCategoryName() ?></h3>
         <div>
-            <?php if (Shop::$current->categoryType === -1): ?>
+            <?php if (Shop::id(Shop::GENERAL_SHOP)->categoryType === -1): ?>
                 Вы можете продать ваши предметы за сущие копейки.
             <?php endif; ?>
         </div>
         <!--Магазин-->
-        <?= Shop::$current->getItemsList() ?>
+        <?= Shop::id(Shop::GENERAL_SHOP)->getItemsList() ?>
     </div>
     <div class="right column">
         <div>
             <strong>
                 Масса всех вещей: <?= InventoryItem::getWeightData() ?> <br>
-                Деньги: <?= User::$current->getMoney() ?>.
+                Деньги: <?= User::getInstance()->getMoney() ?>.
             </strong>
         </div>
         <hr>