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/README.md

@@ -7,3 +7,4 @@ All scripts should be executed from the project folder (repository root).
 * **setup** installation script for development setup
 * **update** update script for development or production (--prod) setup
 * **check** setup checking script for development and production (--prod) setup
+* **json_export** export script for development and production (--prod) -- can be used to export all database items belonging to a given event
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