Islandora 957 Embargoed Object Thumbnail display

Relates to the issues described in https://jira.duraspace.org/browse/ISLANDORA-957
This fixes two problems with the logic in handling display of thumbnails for embargoed objects.

It now sets the function to return true when the object is a thumbnail, which is the required return value to allow an object to display.

It also allows thumbnails of embargoed objects to be displayed when the islandora_ip_embargo_embargoed_redirect variable is not set or is empty.