Fix embargoed object  thumbnail datastream display

Fix logic to allow embargoed objects thumbnail datastreams through.
Now returns true if the object being requested is a thumbanail datastream from another objects page, like on a collection display page.

Also allows embargoed object datastreams through even if a redirect URL has not been
set.

Pulls in upstream commit
https://github.com/lyrasis/islandora_ip_embargo/commit/cbbff5ab8e7afe68b6106d446df47e87e51033a0
allows managers to access embargoed objects