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/Tools/SlugGenerator.php

@@ -0,0 +1,51 @@
+<?php namespace BookStack\Entities;
+
+use Illuminate\Support\Str;
+
+class SlugGenerator
+{
+
+    /**
+     * Generate a fresh slug for the given entity.
+     * The slug will generated so it does not conflict within the same parent item.
+     */
+    public function generate(Entity $entity): string
+    {
+        $slug = $this->formatNameAsSlug($entity->name);
+        while ($this->slugInUse($slug, $entity)) {
+            $slug .= '-' . substr(md5(rand(1, 500)), 0, 3);
+        }
+        return $slug;
+    }
+
+    /**
+     * Format a name as a url slug.
+     */
+    protected function formatNameAsSlug(string $name): string
+    {
+        $slug = Str::slug($name);
+        if ($slug === "") {
+            $slug = substr(md5(rand(1, 500)), 0, 5);
+        }
+        return $slug;
+    }
+
+    /**
+     * Check if a slug is already in-use for this
+     * type of model within the same parent.
+     */
+    protected function slugInUse(string $slug, Entity $entity): bool
+    {
+        $query = $entity->newQuery()->where('slug', '=', $slug);
+
+        if ($entity instanceof BookChild) {
+            $query->where('book_id', '=', $entity->book_id);
+        }
+
+        if ($entity->id) {
+            $query->where('id', '!=', $entity->id);
+        }
+
+        return $query->count() > 0;
+    }
+}