Add S3PublicStorage public storage config for S3

Make it possible to more easily configure a
GLAMkit site to use public instead of private
S3 storage.

The implementation is drawn from the languishing
and contentious pull request at
#217

Author: jmurty

icekit/utils/storage.py

@@ -63,4 +63,13 @@ class S3DefaultStorage(
     pass
 
 
+class S3PublicStorage(
+        # HashedMediaMixin,
+        S3MediaLocationMixin,
+        S3PublicMixin,
+        S3Boto3Storage):
+
+    pass
+
+
 default_storage = get_storage_class(settings.DEFAULT_FILE_STORAGE)()