Refactored some core entity actions

- Created BookChild class to share some page/chapter logic.
- Gave entities the power to generate their own permissions and slugs.
- Moved bits out of BaseController constructor since it was overly
sticky.
- Moved slug generation logic into its own class.
- Created a facade for permissions due to high use.
- Fixed failing test issues from last commits

app/Entities/Chapter.php

@@ -1,6 +1,8 @@
 <?php namespace BookStack\Entities;
 
-class Chapter extends Entity
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+class Chapter extends BookChild
 {
     public $searchFactor = 1.3;
 
@@ -15,15 +17,6 @@ class Chapter extends Entity
         return 'BookStack\\Chapter';
     }
 
-    /**
-     * Get the book this chapter is within.
-     * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
-     */
-    public function book()
-    {
-        return $this->belongsTo(Book::class);
-    }
-
     /**
      * Get the pages that this chapter contains.
      * @param string $dir