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.py

+import json
+import sys
+
+event_id = int(sys.argv[1])
+target_name = sys.argv[2]
+
+print(f"Creating export for event '{event_id}' as '{target_name}'")
+
+# Load json file just created by django
+with open('backups/akplanning_only.json', 'r') as json_file:
+    exported_entries = json.load(json_file)
+print(f"Loaded {len(exported_entries)} entries in total, restricting to event...")
+
+entries_without_event = 0
+entries_out = []
+virtual_rooms_to_preserve = set()
+
+# Loop over all dumped entries
+for entry in exported_entries:
+    # Handle all entries with event reference
+    if "event" in entry['fields']:
+        event = int(entry['fields']['event'])
+
+        # Does this entry belong to the event we are looking for?
+        if event == event_id:
+            # Store for backup
+            entries_out.append(entry)
+
+            # Remember the primary keys of all rooms of this event
+            # Required for special handling of virtual rooms,
+            # since they inherit from normal rooms and have no direct event reference
+            if entry['model'] == "AKModel.room":
+                virtual_rooms_to_preserve.add(entry['pk'])
+    # Handle entries without event reference
+    else:
+        # Backup virtual rooms of that event
+        if entry['model'] == "AKOnline.virtualroom":
+            if entry['pk'] in virtual_rooms_to_preserve:
+                entries_out.append(entry)
+        # Backup the event itself
+        elif entry['model'] == "AKModel.event":
+            if int(entry['pk']) == event_id:
+                entries_out.append(entry)
+        # Backup tags
+        elif entry['model'] == "AKModel.aktag":
+            # No restriction needed, backup all tags
+            entries_out.append(entry)
+        else:
+            # This should normally not happen (all other models should have a reference to the event)
+            entries_without_event += 1
+            print(entry)
+
+print(f"Ignored entries without event: {entries_without_event}")
+print(f"Exporting {len(entries_out)} entries for event")
+
+with open(f'backups/{target_name}.json', 'w') as json_file:
+    json.dump(entries_out, json_file, indent=2)