Refactor some backend logic for EP workflow

Some of the backend logic could be simplified or refactored to make it easier to understand and maybe more reliable. For example, the [`publish_public_record` view handler](https://github.com/palkerecsenyi/cds-rdm/blob/ba22c57af46543ef473965211211f2165fff300a/site/cds_rdm/requests/views.py#L83) is currently quite complicated.