fix editable article published date, quick improvements

project/showcase_site/factory.py

project/showcase_site/migrations/0002_auto_20180512_1643.py

project/showcase_site/models.py

project/showcase_site/serializers.py

project/tests/test_showcase_site/test_article.py

project/tests/test_showcase_site/test_article_api.py

project/tests/test_showcase_site/test_partner.py