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/partials/contest_balloon.html

@@ -25,7 +25,7 @@
           </button> 
         </form>
       {% endif %}
-      <b>{{ String.fromCharCode(65+tdoc.pids.indexOf(bdoc.pid)) }}</b>&nbsp;&nbsp;({{ tdoc.balloon[bdoc.pid].name }})
+      <b>{{ utils.getAlphabeticId(tdoc.pids.indexOf(bdoc.pid)) }}</b>&nbsp;&nbsp;({{ tdoc.balloon[bdoc.pid].name }})
     </td>
     <td class="col--submit-by" data-tooltip="{{ bdoc._id.getTimestamp().format() }}">{{ user.render_inline(udict[bdoc.uid], badge=false) }}</td>
     <td class="col--send-by"{% if bdoc.sent %} data-tooltip="{{ bdoc.sentAt.format() }}"{% endif %}>