Add the missing DELETE checkbox to 'Batch end' (#384)

The input slice was (2, 6) due to additional test case fields in DMOJ: `output-prefix`, `output-limit`, and `checker`. These fields do not exist in the VNOJ fork, necessitating this change.

templates/problem/data.html

@@ -277,7 +277,7 @@
                 $order.after($('<span>', {'class': 'order'}).text($order.val()))
                     .after($('<i>', {'class': 'fa fa-fw fa-lg fa-ellipsis-v'}));
 
-                var $opts = $tr.find('input').slice(2, 6);
+                var $opts = $tr.find('input').slice(2, 4);
                 var $files = $tr.find('select').slice(1, 3);
                 var $checker = $files.end().last();
                 $tr.find('select[id$=type]').change(function () {