Bug 28909: Allow illview to use backend template

Bug 28909: Allow illview to use backend template

This commit allows illview to use an intranet template supplied by the backend, as
per other stages. The template must be named illview.inc

Test plan:
- Apply the patch
- Install FreeForm backend (https://github.com/PTFS-Europe/koha-ill-freeform)
- Create a request
- On the resulting "Manage ILL request" note the display is normal
- Create a new file in the FreeForm backend directory: /intra-includes/illview.inc
  with the following content:

  [% BLOCK backend_jsinclude %]
  <script>(function() { alert("Working!"); })()</script>
  [% END %]

- Restart Plack if enabled
- Reload the page
=> TEST: Observe that the alert specified in the template is displayed

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>