Bug 17989: Extend bad template check

Bug 17989: Extend bad template check

The check is now extended by only allowing templates from the regular
Koha template directories and additional plugin directories as defined
in koha-conf.xml.

Note: In order to prevent an uninitialized warning on $theme from sub
themelanguage for a not-existing file, I added a trivial assignment. Will
get further attention in a follow-up.

Test plan:
[1] Run t/db_dependent/Auth.t
[2] Run t/db_dependent/Templates.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>