Moved models to folder, renamed managers to tools

Tools seems to fit better since the classes were a bit of a mixed bunch
and did not always manage.
Also simplified the structure of the SlugGenerator class.
Also focused EntityContext on shelves and simplified to use session
helper.

app/Entities/Models/HasCoverImage.php

@@ -0,0 +1,20 @@
+<?php
+
+
+namespace BookStack\Entities;
+
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+interface HasCoverImage
+{
+
+    /**
+     * 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;
+}