Block repeated requests to register the same dataset

After submitting a request and before the dataset is finalised, we should have a way to block repeated requests to register the same repository. As it is now, if a user reloads the final request confirmation page, they will trigger a second request as the page loads.

The service should keep a list of ongoing registration jobs and when the user attempts to register a repository while a request is still being processed, they should get an appropriate message.

Trace the registration request flow from the final request confirmation page, focusing on how repository registration jobs are submitted and how responses are shown. Verify the reload scenario and the case where a repository is already being processed. Done means repeated in-progress registrations are blocked and the user receives an appropriate message.

Written by the indexing model from the issue text.