Introduce utility to export all data related to an event to json

This adds such a utility.

Additionally, add direct event references to AKOrgaMessages for easier management. This includes a migration that makes sure all existing messages are updated accordingly.

AKModel/migrations/0050_message_event_reference.py

+# Generated by Django 3.1.8 on 2022-05-12 16:57
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+def forwards_func(apps, schema_editor):
+    # Set event to the corresponding even (from the AK) each
+    AKOrgaMessage = apps.get_model("AKModel", "AKOrgaMessage")
+    for message in AKOrgaMessage.objects.all():
+        message.event = message.ak.event
+        message.save()
+
+
+def reverse_func(apps, schema_editor):
+    # No need to do something here, field will be deleted anyway
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('AKModel', '0049_interest_window'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='akorgamessage',
+            name='event',
+            field=models.ForeignKey(blank=True, help_text='Associated event', null=True,
+                                    on_delete=django.db.models.deletion.CASCADE, to='AKModel.event',
+                                    verbose_name='Event'),
+        ),
+        migrations.RunPython(forwards_func, reverse_func),
+        migrations.AlterField(
+            model_name='akorgamessage',
+            name='event',
+            field=models.ForeignKey(help_text='Associated event', on_delete=django.db.models.deletion.CASCADE,
+                                    to='AKModel.event', verbose_name='Event'),
+        ),
+    ]