Merge branch 's3-thumbnails' into 'master'

.gitlab-ci.yml

VERSION

arkindex/dataimport/tests/test_iiif.py

arkindex/documents/models.py

...ndex/documents/tests/commands/test_generate_thumbnails.py

arkindex/images/models.py

arkindex/images/tests/test_image.py

arkindex/images/tests/test_thumbnail.py

arkindex/project/aws.py

arkindex/project/checks.py

arkindex/project/settings.py

arkindex/project/tests/test_checks.py

requirements.txt