Maintenance: Continued work towards PHPstan level 2

Updated html description code to be behind a proper interface.
Set new convention for mode traits/interfaces.

app/Entities/Models/HtmlDescriptionInterface.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace BookStack\Entities\Models;
+
+interface HtmlDescriptionInterface
+{
+    /**
+     * Get the HTML-based description for this item.
+     * By default, the content should be sanitised unless raw is set to true.
+     */
+    public function descriptionHtml(bool $raw = false): string;
+
+    /**
+     * Set the HTML-based description for this item.
+     */
+    public function setDescriptionHtml(string $html, string|null $plaintext = null): void;
+}