core&ui: fix contest with more than 26 problems have wrong alphabetic id (#1008)

Co-authored-by: undefined <i@undefined.moe>

packages/ui-default/templates/contest_problemlist.html

@@ -185,7 +185,7 @@
                   <option value="0">{{ _('General Issue') }}</option>
                   <option value="-1">{{ _('Technical Issue') }}</option>
                   {% for pid in tdoc.pids %}
-                  <option value="{{ pid }}">{{ String.fromCharCode(65+loop.index0) }}. {{ pdict[pid].title }}</option>
+                  <option value="{{ pid }}">{{ utils.getAlphabeticId(loop.index0) }}. {{ pdict[pid].title }}</option>
                   {% endfor %}
                 </select>
               </label>