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/BookChild.php

@@ -0,0 +1,57 @@
+<?php namespace BookStack\Entities;
+
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * Class BookChild
+ * @property int $book_id
+ * @property int $priority
+ * @property Book $book
+ * @method Builder whereSlugs(string $bookSlug, string $childSlug)
+ */
+abstract class BookChild extends Entity
+{
+
+    /**
+     * Scope a query to find items where the the child has the given childSlug
+     * where its parent has the bookSlug.
+     */
+    public function scopeWhereSlugs(Builder $query, string $bookSlug, string $childSlug)
+    {
+        return $query->with('book')
+            ->whereHas('book', function (Builder $query) use ($bookSlug) {
+                $query->where('slug', '=', $bookSlug);
+            })
+            ->where('slug', '=', $childSlug);
+    }
+
+    /**
+     * Get the book this page sits in.
+     * @return BelongsTo
+     */
+    public function book(): BelongsTo
+    {
+        return $this->belongsTo(Book::class);
+    }
+
+    /**
+     * Change the book that this entity belongs to.
+     */
+    public function changeBook(int $newBookId): Entity
+    {
+        $this->book_id = $newBookId;
+        $this->refreshSlug();
+        $this->save();
+        $this->refresh();
+
+        // Update all child pages if a chapter
+        if ($this instanceof Chapter) {
+            foreach ($this->pages as $page) {
+                $page->changeBook($newBookId);
+            }
+        }
+
+        return $this;
+    }
+}