Updates to existing requests

The service should keep a record of open requests and block the user from resubmitting a request from the same repository while a request is still open **unless** the source repository has changed. The record should be a database (or a flat file) with the following information:
- Repository ID
- Reserved/assigned DOI
- Commit hash

When a new request comes in, the workflow would then be:
- Check if the repository ID is already in the file.
- If it's not, it's a new request and the request continues normally.
- If the repository ID is in the file and the commit hash is the same, inform the user that the request is still pending and don't start a new registration.
- If the commit hash is different, update the existing request with the new data and inform the user that their request has been updated without changing the reserved DOI.