Bug 7317: Handle backend absense more gracefuly

Bug 7317: Handle backend absense more gracefuly

5/ This patch makes Koha::Illrequest->load_backend raise an exception
if the passed backend is invalid. This way we will catch more errors introduced.

The patch also disables the 'New Ill request' when no backends are available. Gets
rid of a related warnings.

Both OPAC and Intranet now display a warning message when no backends
are available.

Tests are added for the load_backend changes.

4/ This patch fixes the path for the checkboxes jquery plugin, and removes the include
for tablesorter, as this implementation uses Datatables. This is obviously code for older
Koha, ported to master.

TODO: There's something wrong on the styling. My idea is to get rid
of the custom column visualization tool, and have it display as regular
DataTables. We can then introduce the use of colvis on a separate bug
report.

Note: POD coverage for the exceptions file is wrongly tested. It is a false positive.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>