Add support for showing parent groups instead of original groups

if there is only one group and this group has parent groups

Advance #71

aleksis/apps/chronos/settings.py

 from django.utils.translation import gettext_lazy as _
 
 CONSTANCE_CONFIG = {
+    "CHRONOS_USE_PARENT_GROUPS": (
+        False,
+        _(
+            "If an lesson or substitution has only one group"
+            " and this group has parent groups,"
+            " show the parent groups instead of the original group."
+        ),
+    ),
     "CHRONOS_SUBSTITUTIONS_PRINT_DAY_NUMBER": (
         2,
         _("Number of days shown on substitutions print view"),
@@ -12,6 +20,7 @@ CONSTANCE_CONFIG = {
 }
 CONSTANCE_CONFIG_FIELDSETS = {
     "Chronos settings": (
+        "CHRONOS_USE_PARENT_GROUPS",
         "CHRONOS_SUBSTITUTIONS_PRINT_DAY_NUMBER",
         "CHRONOS_SUBSTITUTIONS_SHOW_HEADER_BOX",
     ),

aleksis/apps/chronos/templates/chronos/partials/groups.html

-{% for group in groups %}
-  <a href="{% url "timetable" "group" group.pk %}">
-    {{ group.short_name }}{% if not forloop.last %},{% endif %}
-  </a>
-{% endfor %}
+{% if groups.count == 1 and groups.0.parent_groups.all and config.CHRONOS_USE_PARENT_GROUPS %}
+  {% include "chronos/partials/groups_part.html" with groups=groups.0.parent_groups.all %}
+{% else %}
+  {% include "chronos/partials/groups_part.html" with groups=groups %}
+{% endif %}

aleksis/apps/chronos/templates/chronos/partials/groups_part.html

+{% for group in groups %}
+  <a href="{% url "timetable" "group" group.pk %}">
+    {{ group.short_name }}{% if not forloop.last %},{% endif %}
+  </a>
+{% endfor %}