HOLI-8934: add feed post images

feed_posts/helpers.py

feed_posts/migrations/0014_feedpostimage.py

feed_posts/models.py

feed_posts/repositories.py

feed_posts/schema/mutations.py

feed_posts/schema/types.py

feed_posts/serializers.py

feed_posts/services.py

feed_posts/tests/test_graphql.py

openbook/settings/__init__.py