job-server API does not allow re-uploading already uploaded files.

[bloodearnest]

This was a design decision from a long time ago that needs undoing.

We probably want to allow duplicate releases uploads, for convenience.

There's some potential consequences if we do:

• checkers may spend time checking the same file multiple times. We could maybe instead auto-approve already released files, so they are released as part of the request, but don't require any review.

• It might make redaction in job-server a bit more complex, as we'd need to redact all instances of an uploaded file.