Added ability to secure images behind auth

Still in testing.
Adds STORAGE_TYPE=local_secure option for setting images to be behind
auth. Stores images alongside attachments in /storage/uploads/images.

app/Http/Controllers/ImageController.php

@@ -1,6 +1,7 @@
 <?php namespace BookStack\Http\Controllers;
 
 use BookStack\Exceptions\ImageUploadException;
+use BookStack\Exceptions\NotFoundException;
 use BookStack\Repos\EntityRepo;
 use BookStack\Repos\ImageRepo;
 use Illuminate\Filesystem\Filesystem as File;
@@ -28,6 +29,21 @@ class ImageController extends Controller
         parent::__construct();
     }
 
+    /**
+     * Provide an image file from storage.
+     * @param string $path
+     * @return mixed
+     */
+    public function showImage(string $path)
+    {
+        $path = storage_path('uploads/images/' . $path);
+        if (!file_exists($path)) {
+            abort(404);
+        }
+
+        return response()->file($path);
+    }
+
     /**
      * Get all images for a specific type, Paginated
      * @param string $type