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/Bookshelf.php

@@ -0,0 +1,122 @@
+<?php namespace BookStack\Entities;
+
+use BookStack\Uploads\Image;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+
+class Bookshelf extends Entity implements HasCoverImage
+{
+    protected $table = 'bookshelves';
+
+    public $searchFactor = 3;
+
+    protected $fillable = ['name', 'description', 'image_id'];
+
+    protected $hidden = ['restricted', 'image_id'];
+
+    /**
+     * Get the books in this shelf.
+     * Should not be used directly since does not take into account permissions.
+     * @return \Illuminate\Database\Eloquent\Relations\BelongsToMany
+     */
+    public function books()
+    {
+        return $this->belongsToMany(Book::class, 'bookshelves_books', 'bookshelf_id', 'book_id')
+            ->withPivot('order')
+            ->orderBy('order', 'asc');
+    }
+
+    /**
+     * Related books that are visible to the current user.
+     */
+    public function visibleBooks(): BelongsToMany
+    {
+        return $this->books()->visible();
+    }
+
+    /**
+     * Get the url for this bookshelf.
+     * @param string|bool $path
+     * @return string
+     */
+    public function getUrl($path = false)
+    {
+        if ($path !== false) {
+            return url('/shelves/' . urlencode($this->slug) . '/' . trim($path, '/'));
+        }
+        return url('/shelves/' . urlencode($this->slug));
+    }
+
+    /**
+     * Returns BookShelf cover image, if cover does 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)
+    {
+        // TODO - Make generic, focused on books right now, Perhaps set-up a better image
+        $default = 'data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==';
+        if (!$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 shelf
+     */
+    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_shelf';
+    }
+
+    /**
+     * 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;
+    }
+
+    /**
+     * Check if this shelf contains the given book.
+     * @param Book $book
+     * @return bool
+     */
+    public function contains(Book $book): bool
+    {
+        return $this->books()->where('id', '=', $book->id)->count() > 0;
+    }
+
+    /**
+     * Add a book to the end of this shelf.
+     * @param Book $book
+     */
+    public function appendBook(Book $book)
+    {
+        if ($this->contains($book)) {
+            return;
+        }
+
+        $maxOrder = $this->books()->max('order');
+        $this->books()->attach($book->id, ['order' => $maxOrder + 1]);
+    }
+}