Chat data-model reform:
- introduce on delete cascade on foreign key constraints so package can be removed easily e.g. from the subsite UI
- remove remaining stored procedures (and function_args definitions) to relieve the burden on the development process and reduce divergency between oracle and postgres code
- converge oracle and postgres datamodels:
. oracle -> add missing messages_time_window column
. postgres -> remove creation_date from chat_transcripts, which was found to be still existing on old downstream installations
