[Merged] GPDR, registrations, e-mails

[Didayolo]

Registrations and un-registrations:

• [x] #2319
• [ ] #2252
• [x] #2432
• [ ] #2104
• [x] #2528

Issues with e-mails:

• [ ] #2412
• [ ] #2477
• [ ] #2449
• [ ] #2765

[Didayolo]

As mentioned in #2686:

Adding admins to competitions should be done with email address, not username. Also, it should not give the email information.

--> solved because it's a super-user feature