Bug 23804: Correctly catch remote itemtype images

Bug 23804: Correctly catch remote itemtype images

Bug 20216 missed the mark and although it did start display a URL for
the itemtype image in use, it was not limited to remote urls and as such
broke the checked handling for displaying which local image was
selected.

Test plan:
1) Select a local image for an itemtype.
2) Check the correct image is displayed in the admin table.
3) Edit the itemtype checking that the correct image is checked.
4) Change to a remote image for the itemtype
5) Check the correct image is displayed in the admin table.
6) Edit the itemtype again and check that the URL still displays and is
   checked in the 'Remote image' tab.
7) Signoff

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>