Add title to link back to exhibit page

Code now iterates thru the exhibit pages that contain the item,
and checks that $_SERVER['HTTP_REFERER'] points to one of these
pages. If so, the link will print, using as text the title of
the exhibit page.
Added/Modified code for link back to exhibit page(s)

Modified code that creates the link(s) back to the exhibit
pages. This includes removing some code that is now in the
CulCustomize plugin, and subsequently added calls to 3 functions
defined in CulCustomize. Also, the links to the other exhibit
pages containing the current item now appear at the top of the
item page.