Refactored sort system a little

To standardise the handled data format a little better.

app/Entities/Tools/BookSortMap.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace BookStack\Entities\Tools;
+
+class BookSortMap
+{
+    /**
+     * @var BookSortMapItem[]
+     */
+    protected $mapData = [];
+
+    public function addItem(BookSortMapItem $mapItem): void
+    {
+        $this->mapData[] = $mapItem;
+    }
+
+    /**
+     * @return BookSortMapItem[]
+     */
+    public function all(): array
+    {
+        return $this->mapData;
+    }
+
+    public static function fromJson(string $json): self
+    {
+        $map = new static();
+        $mapData = json_decode($json);
+
+        foreach ($mapData as $mapDataItem) {
+            $item = new BookSortMapItem(
+                intval($mapDataItem->id),
+                intval($mapDataItem->sort),
+                $mapDataItem->parentChapter ? intval($mapDataItem->parentChapter) : null,
+                $mapDataItem->type,
+                intval($mapDataItem->book)
+            );
+
+            $map->addItem($item);
+        }
+
+        return $map;
+    }
+
+}