Grunt: update generate_themes task to use new DownloadBuilder API

- DownloadBuilder accepts the config file as an option. It's not necessary
  to workaround it by changing the DownloadBuilder config file anymore;
- DownloadBuilder has exposed a different method to generate the gallery:
  download.themegallery(), instead of frontend.download.themegallery();
- Changed theme.fetchImages -> theme.generateImages