Added reference storage system, and command to re-index

Also re-named/orgranized some files for this, to make them "References"
specific instead of a subset of "Util".

app/References/ModelResolvers/BookLinkModelResolver.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace BookStack\References\ModelResolvers;
+
+use BookStack\Entities\Models\Book;
+use BookStack\Model;
+
+class BookLinkModelResolver implements CrossLinkModelResolver
+{
+    public function resolve(string $link): ?Model
+    {
+        $pattern = '/^' . preg_quote(url('/books'), '/') . '\/([\w-]+)' . '([#?\/]|$)/';
+        $matches = [];
+        $match = preg_match($pattern, $link, $matches);
+        if (!$match) {
+            return null;
+        }
+
+        $bookSlug = $matches[1];
+
+        /** @var ?Book $model */
+        $model = Book::query()->where('slug', '=',  $bookSlug)->first();
+
+        return $model;
+    }
+}