Bug 18010: Remove potential exposure from gettemplate

Bug 18010: Remove potential exposure from gettemplate

A similar bad template check from C4::Auth::get_template_and_user
should be applied in C4::Templates::gettemplate.

Before this patch it would be possible to expose files like:
my $template = C4::Templates::gettemplate(
    '/etc/passwd', 'intranet', CGI::new, 1
);
print $template->output;

Note that the is_plugin flag in the above call is the culprit. This patch
provides a quick security fix without touching get_template_and_user, and
can be backported to stable branches.
I will provide an enhanced and centralized check on report 17989, also
removing the is_plugin flag.

Note: We allow .pref here too for use in admin/preferences.pl.

Test plan:
[1] Run t/db_dependent/Auth.t (triggering get_template_and_user and
    gettemplate).
[2] Run t/db_dependent/Templates.t again (see first test plan).
    The tests should no longer fail.
[3] Open a page on opac or intranet.
[4] Open a systempreferences tab.
[5] Add a book to the cart and send it ([opac-]sendbasket uses gettemplate).

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>