Bug 7317: Interlibrary loans framework for Koha.

Bug 7317: Interlibrary loans framework for Koha.

This Commit is at the heart of adding an interlibrary loans framework
for Koha.  The framework does not prescribe a particular workflow.
Instead it provides a general framework that can be extended &
implemented by individual backends whose responsibility it is to
implement a specific workflow.

The module is largely self-sufficient: it adds new tables to the Koha
database and touches only a few files in the Koha source tree.

Primarily, we add our files to the Makefile and the koha-conf.xml,
define ill paths for the REST API, and introduce links from the main
intranet, opac pages & user permissions.

Outside of this we simply add new files & functionality.

Signed-off-by: Magnus Enger <magnus@libriotech.no>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Benjamin Rokseth <benjamin.rokseth@kul.oslo.kommune.no>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>