manifest.py - Get URLs from staticfiles_storage to ensure hash is present

Getting the URL for the file form staticfiles_storage.url() ensures that the correct version hash is in the URL of the file within the appcache manifest.

Note that this may not be the best way to implement this in Pipeline. Please feel free to implement a better solution, but unfortunately I may not be able to give much time to this pull request in the near future (my apologies!)