Fix: backend handling resuming contributions

[zwilling]

Description

During the Galactica Network Ceremony multiple users reported issues of not being able to contribute, even after retrying multiple times.
After some debugging, I came to the conclusion that they were unable to resume contributions because the backend did not allow uploading a new contribution.
When the user was interrupted for any reason during or after the uploading step, a retry generated new random input and therefore a new contribution that needed to be uploaded.

This PR fixes the issue by allowing another upload during the uploading step and handling the consequences during verification.

How Has This Been Tested?

• Deployment finished without errors
• Running the unittests yarn test:prod
• Completing the Galactica Network Ceremony

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (Is there something to change?)
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• I reviewed the code of conduct and contributors' guide