Join codes

[ThatXliner]

Prerequisites

• [X] I have searched the issues and believe that it has not already been requested (and possibly rejected)
• [X] I have made sure that this feature isn't possible with this project's plugin system and/or configuration; core changes must be made to make this happen.
• [X] I agree to follow the Code of Conduct

Feature description

Just like how we have unique codes for attendance, we should have unique codes for joins. But this means that we should probably share some code between these 2 actions, which means that implementing this before #7 would be counterintuitive

• Blocked by #7

[ThatXliner]

Actually, I can implement this partially without #7