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/Reference.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace BookStack\References;
+
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\MorphTo;
+
+/**
+ * @property int $from_id
+ * @property string $from_type
+ * @property int $to_id
+ * @property string $to_type
+ */
+class Reference extends Model
+{
+    public function from(): MorphTo
+    {
+        return $this->morphTo('from');
+    }
+
+    public function to(): MorphTo
+    {
+        return $this->morphTo('to');
+    }
+}