Bug 13948: Add ability to dump template toolkit variables to html comment

Bug 13948: Add ability to dump template toolkit variables to html comment

It would be incredibly helpful if we could easily enable Koha to dump
all Template Toolkit variables to a comment for debugging purposes.

Test Plan:
1) Apply this patch
2) Run updatedatabase
3) Enable the new system preferences DumpTemplateVarsIntranet and
   DumpTemplateVarsOpac
4) Load a page in the staff intranet, view the html source
5) Note the template toolkit variables are embedded in an html comment
6) Load a page in the opac, view the html source
7) Note the template toolkit variable are embedded in an html comment

NOTE: I had to cpan2deb Template::Plugin::Stash to test.
      This is not optimal. Additionally:
      http://www.template-toolkit.org/docs/modules/Template/Plugin/index.html
      does not contain Stash. I suspect this was how it was
      introduced initially by TT.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>