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/CoverImageInterface.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace BookStack\Entities\Models;
+
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+interface CoverImageInterface
+{
+    /**
+     * Get the cover image for this item.
+     */
+    public function cover(): BelongsTo;
+
+    /**
+     * Get the type of the image model that is used when storing a cover image.
+     */
+    public function coverImageTypeKey(): string;
+}