Bug 13473 - Plugins fail

When trying to execute or configure the plugin on 3.18 I got:

Template process failed: file error - doc-head-open.inc: not found at
/usr/share/koha/lib/C4/Templates.pm line 129.

Test Plan:
1) Install the Kitchen Sink plugin
2) Go to the configuration page of the plugin
3) Note the template processing error
4) Apply this patch
5) Refresh the page
6) Note the page now loads

Signed-off-by: Nick <nick@quecheelibrary.org>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit 003993d547)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>

C4/Templates.pm

@@ -302,6 +302,10 @@ sub themelanguage {
             return ( $theme, 'en', uniq( \@themes ) );
         }
     }
+    # tmpl is a full path, so this is a template for a plugin
+    if ( $tmpl =~ /^\// && -e $tmpl ) {
+        return ( $themes[0], $lang, uniq( \@themes ) );
+    }
 }