Bug 18423 - Add child button not always appearing - problem in template variable

Bug 18423 - Add child button not always appearing - problem in template variable

This patch removes the retrieval of the syspref borrowerRelationship
from the scripts and moves it to a check using the Template Toolkit
plugin

To test:
1 - Apply patch
2 - Ensure 'Add child' button displays appropriately on all member pages.

Tested 3 pateches together, works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>