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/Book.php

@@ -0,0 +1,130 @@
+<?php namespace BookStack\Entities;
+
+use BookStack\Uploads\Image;
+use Exception;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+use Illuminate\Support\Collection;
+
+/**
+ * Class Book
+ * @property string $description
+ * @property int $image_id
+ * @property Image|null $cover
+ */
+class Book extends Entity implements HasCoverImage
+{
+    public $searchFactor = 2;
+
+    protected $fillable = ['name', 'description'];
+    protected $hidden = ['restricted', 'pivot', 'image_id'];
+
+    /**
+     * Get the url for this book.
+     * @param string|bool $path
+     * @return string
+     */
+    public function getUrl($path = false)
+    {
+        if ($path !== false) {
+            return url('/books/' . urlencode($this->slug) . '/' . trim($path, '/'));
+        }
+        return url('/books/' . urlencode($this->slug));
+    }
+
+    /**
+     * Returns book cover image, if book cover not exists return default cover image.
+     * @param int $width - Width of the image
+     * @param int $height - Height of the image
+     * @return string
+     */
+    public function getBookCover($width = 440, $height = 250)
+    {
+        $default = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
+        if (!$this->image_id) {
+            return $default;
+        }
+
+        try {
+            $cover = $this->cover ? url($this->cover->getThumb($width, $height, false)) : $default;
+        } catch (Exception $err) {
+            $cover = $default;
+        }
+        return $cover;
+    }
+
+    /**
+     * Get the cover image of the book
+     */
+    public function cover(): BelongsTo
+    {
+        return $this->belongsTo(Image::class, 'image_id');
+    }
+
+    /**
+     * Get the type of the image model that is used when storing a cover image.
+     */
+    public function coverImageTypeKey(): string
+    {
+        return 'cover_book';
+    }
+
+    /**
+     * Get all pages within this book.
+     * @return HasMany
+     */
+    public function pages()
+    {
+        return $this->hasMany(Page::class);
+    }
+
+    /**
+     * Get the direct child pages of this book.
+     * @return HasMany
+     */
+    public function directPages()
+    {
+        return $this->pages()->where('chapter_id', '=', '0');
+    }
+
+    /**
+     * Get all chapters within this book.
+     * @return HasMany
+     */
+    public function chapters()
+    {
+        return $this->hasMany(Chapter::class);
+    }
+
+    /**
+     * Get the shelves this book is contained within.
+     * @return BelongsToMany
+     */
+    public function shelves()
+    {
+        return $this->belongsToMany(Bookshelf::class, 'bookshelves_books', 'book_id', 'bookshelf_id');
+    }
+
+    /**
+     * Get the direct child items within this book.
+     * @return Collection
+     */
+    public function getDirectChildren(): Collection
+    {
+        $pages = $this->directPages()->visible()->get();
+        $chapters = $this->chapters()->visible()->get();
+        return $pages->concat($chapters)->sortBy('priority')->sortByDesc('draft');
+    }
+
+    /**
+     * Get an excerpt of this book's description to the specified length or less.
+     * @param int $length
+     * @return string
+     */
+    public function getExcerpt(int $length = 100)
+    {
+        $description = $this->description;
+        return mb_strlen($description) > $length ? mb_substr($description, 0, $length-3) . '...' : $description;
+    }
+}