Fix missing template import.

biscuit/apps/alsijil/templates/alsijil/group_week.html

 {# -*- engine:django -*- #}
 {% extends "core/base.html" %}
-{% load bootstrap4 font_awesome i18n %}
+{% load bootstrap4 font_awesome i18n week_helpers %}
 
 {% block page_title %}
  {% blocktrans %}Current week{% endblocktrans %}