core&ui: add keep scoreboard hidden option

packages/ui-default/templates/contest_edit.html

@@ -174,6 +174,17 @@
               value:tdoc.allowPrint|default(false),
               row:false
             }) }}
+            <div style="display: none" class="contest-rule-settings contest-rule--oi contest-rule--strictioi">
+            {{ form.form_checkbox({
+              columns:4,
+              label:'Keep Scoreboard Hidden',
+              name:'keepScoreboardHidden',
+              placeholder:_('Keep scoreboard hidden even after contest ends'),
+              value:tdoc.keepScoreboardHidden|default(false),
+              disabled:true,
+              row:false
+            }) }}
+            </div>
             <div style="display: none" class="contest-rule-settings contest-rule--acm contest-rule--ioi">
             {{ form.form_text({
               columns:4,