Thumbnails: Added OOM handling and regen endpoint

- Added some level of app out-of-memory handling so we can show a proper
  error message upon OOM events.
- Added endpoint and image-manager button/action for regenerating
  thumbnails for an image so they can be re-created upon failure.

app/Uploads/Controllers/ImageController.php

@@ -8,6 +8,7 @@ use BookStack\Http\Controller;
 use BookStack\Uploads\Image;
 use BookStack\Uploads\ImageRepo;
 use BookStack\Uploads\ImageService;
+use BookStack\Util\OutOfMemoryHandler;
 use Exception;
 use Illuminate\Http\Request;
 use Illuminate\Validation\ValidationException;
@@ -121,6 +122,24 @@ class ImageController extends Controller
         return response('');
     }
 
+    /**
+     * Rebuild the thumbnails for the given image.
+     */
+    public function rebuildThumbnails(string $id)
+    {
+        $image = $this->imageRepo->getById($id);
+        $this->checkImagePermission($image);
+        $this->checkOwnablePermission('image-update', $image);
+
+        new OutOfMemoryHandler(function () {
+            return $this->jsonError(trans('errors.image_thumbnail_memory_limit'));
+        });
+
+        $this->imageRepo->loadThumbs($image, true);
+
+        return response(trans('components.image_rebuild_thumbs_success'));
+    }
+
     /**
      * Check related page permission and ensure type is drawio or gallery.
      */