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/Chapter.php

@@ -0,0 +1,73 @@
+<?php namespace BookStack\Entities;
+
+use Illuminate\Support\Collection;
+
+/**
+ * Class Chapter
+ * @property Collection<Page> $pages
+ */
+class Chapter extends BookChild
+{
+    public $searchFactor = 1.3;
+
+    protected $fillable = ['name', 'description', 'priority', 'book_id'];
+    protected $hidden = ['restricted', 'pivot'];
+
+    /**
+     * Get the pages that this chapter contains.
+     * @param string $dir
+     * @return mixed
+     */
+    public function pages($dir = 'ASC')
+    {
+        return $this->hasMany(Page::class)->orderBy('priority', $dir);
+    }
+
+    /**
+     * Get the url of this chapter.
+     * @param string|bool $path
+     * @return string
+     */
+    public function getUrl($path = false)
+    {
+        $bookSlug = $this->getAttribute('bookSlug') ? $this->getAttribute('bookSlug') : $this->book->slug;
+        $fullPath = '/books/' . urlencode($bookSlug) . '/chapter/' . urlencode($this->slug);
+
+        if ($path !== false) {
+            $fullPath .= '/' . trim($path, '/');
+        }
+
+        return url($fullPath);
+    }
+
+    /**
+     * Get an excerpt of this chapter's description to the specified length or less.
+     * @param int $length
+     * @return string
+     */
+    public function getExcerpt(int $length = 100)
+    {
+        $description = $this->text ?? $this->description;
+        return mb_strlen($description) > $length ? mb_substr($description, 0, $length-3) . '...' : $description;
+    }
+
+    /**
+     * Check if this chapter has any child pages.
+     * @return bool
+     */
+    public function hasChildren()
+    {
+        return count($this->pages) > 0;
+    }
+
+    /**
+     * Get the visible pages in this chapter.
+     */
+    public function getVisiblePages(): Collection
+    {
+        return $this->pages()->visible()
+        ->orderBy('draft', 'desc')
+        ->orderBy('priority', 'asc')
+        ->get();
+    }
+}