Bug 22818: (follow-up) Respond to feedback

Bug 22818: (follow-up) Respond to feedback

This commit fixes this bug, it was broken in a number of ways.

Fixes include:

- Added necessary config block in C4::Letters to enable the TT notice
syntax introduced in an earlier commit to work
- Changed template variables to refer to singular objects rather than
multiple e.g. borrowers -> borrower
- Fixed missing / misnamed variables

This commit also implements the additional syspref checks suggested by
Katrin in comment #87

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>