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.

Utils/json_export.sh

+#!/usr/bin/env bash
+# Update AKPlanning
+# execute as Utils/update.sh id_to_export target_name_to_export_to [--prod]
+
+# abort on error, print executed commands
+set -ex
+
+# activate virtualenv if necessary
+if [ -z ${VIRTUAL_ENV+x} ]; then
+    source venv/bin/activate
+fi
+
+# set environment variable when we want to update in production
+if [ "$3" = "--prod" ]; then
+    export DJANGO_SETTINGS_MODULE=AKPlanning.settings_production
+fi
+
+mkdir -p ../backups/
+python manage.py dumpdata AKDashboard AKModel AKOnline AKPlan AKScheduling AKSubmission --indent=2 > "backups/akplanning_only.json" --traceback
+
+python ./Utils/json_export.py $1 $2
+
+rm backups/akplanning_only.json