Support re-joining the server

From a post-conference support request at EP2024: A user left the server after the conference, but wanted to re-join to access some shared URLs.

Requirement

Users shall be able to re-join the server with the same ticket details and the same Discord account.

Suggested implementation

We currently store a list of all tickets used for registration to prevent multiple users from registering with the same ticket.

Let's add the Discord account ID to these records, and only prevent re-registrations with different account IDs.