Bug 29172: Make CustomCoverImagesURL compatible with control fields

Bug 29172: Make CustomCoverImagesURL compatible with control fields

Bug 22445 added CustomCoverImagesURL that deals with MARC fields, but
control fields were not supported.

Test plan:
Turn CustomCoverImages on
Fill CustomCoverImagesURL with https://covers.openlibrary.org/b/isbn/{006}-M.jpg
Add an isbn in 006 (0596001738 will work)
Go to the detail page of the bibliographic record and confirm that you
see a cover image.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>