Referral program during registration

We need to create a referral program for the applications, this would be especially good for Connected but probably won't have the time to get there.

@pholm @Earlo I'd love your input on this. How I imagine it:

* User clicks register to the event
* User fills in information
* User sees "Refer 10 friends and secure your spot in hub"
* User refers friend, and when friend registers -> User gets point -> 10 points earn a secured spot

Comments?