Block repeated requests to register the same dataset

[achilleas-k]

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.