Bug 22445: Add new prefs *CustomCoverImages*

Bug 22445: Add new prefs *CustomCoverImages*

This patch set adds a new method to display cover images for
bibliographic records.
The idea is to provide a flexible way allowing several use cases:
- local path
- remote url
- webservice
which would follow a given pattern.

So far only the following patterns are available:
- {isbn} will be replaced byt the isbn of the record
- {issn}
- using MARC subfields:
  {024$a} to replace with the value in 024$a
(Note that the selector has been changed from %% to {} in a coming
patch)

For example
- /your/local/images/{isbn}.png
- https://covers.openlibrary.org/b/isbn/{isbn}-M.jpb
- etc.

There are 3 new sysprefs:
CustomCoverImagesURL
=> Define the path/url
CustomCoverImages
=> Turn on/off the display of the custom cover image in the staff client
OPACCustomCoverImages
=> Turn on/off the display of the custom cover image at the OPAC

Sponsored-by: Orex Digital
Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
Signed-off-by: Hugo Agud <hagud@orex.es>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Kyle Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>