Implement Direct Referral System

[limwa]

A referral system will be needed for ENEI 2025. Ideally, a promotor will have a link through which people can sign up for the event.

At the end of the signup process, there is an indication of which promotor will receive points and how many points will be awarded to them. For each participant, the promotor who referred them should be stored in the database (for analytics purposes).

[limwa]

@Naapperas @AvilaAndre