Bug 36244: Do template toolkit processing first

Bug 36244: Do template toolkit processing first

To avoid injection of template toolkit code
from database fields that are controlled by
untrusted sources.

Test plan:

* review subtest 'Template toolkit syntax in
  parameters' in t/db_dependent/Letters.t
* Run the unit test:
  prove t/db_dependent/Letters.t

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>