Create validation to prevent illegal filenames and create migration script to cleanup bad file objects

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Sentry Issue: STUDIO-HEZ

FileNotFoundError: storage/7/8/780014-1473318347-25-20-2.9 not found
  File "contentcuration/utils/publish.py", line 598, in create_perseus_zip
    write_assessment_item(question, zf, channel_id)
  File "contentcuration/utils/publish.py", line 636, in write_assessment_item
    question, question_images = process_image_strings(question, zf, channel_id)
  File "contentcuration/utils/publish.py", line 713, in process_image_strings
    with storage.open(ccmodels.generate_object_storage_name(checksum, filename), 'rb') as imgfile:
  File "contentcuration/utils/gcs_storage.py", line 245, in open
    return self._get_readable_backend(name).open(name, mode)
  File "contentcuration/utils/gcs_storage.py", line 239, in _get_readable_backend
    raise FileNotFoundError("{} not found".format(name))