Prevent an user from changing their documents once they have been validated

[catileptic]

The elections that would have required this modification have been called off. Please do not attempt to solve this Issue.

We're keeping it open in order to server as a reminder of the changes we have planned for.

---

Work in progress (@overeha ):

A user should not be able to change a set of documents that have been used to validate their ability to vote or their candidate. This will prevent a user for uploading one document in order to pass validation and then switch it out for another document.