Bug 37488: Validate paths in datalink.txt/idlink.txt files

Bug 37488: Validate paths in datalink.txt/idlink.txt files

This change validates the paths in datalink.txt/idlink.txt,
so that only images in the unpacked archive directory are allowed

Test plan:
0. Apply the patch
1. koha-plack --reload kohadev
2. Create a datalink.txt file with the following:
42,selfie.jpg
3. Create a jpeg at selfie.jpg
4. ZIP the datalink.txt and selfie.jpg files
5. Upload to the "Upload patron images" tool
(after enabling the "patronimages" system preference)
6. Note that the image uploads correctly

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>