Turn registration numbers into something that can't be mistaken for a sequential number

[ReneB]

After the first round of registrations, we received quite a number of questions from people who thought their registration number was their spot on the waiting list. This can be easily remedied by, for example, substituting the numbers with letters in the mail and interface (could even be as simple as a=1, b=2 or what have you).

[matthijskooijman]

Or maybe remove the registration number entirely. Especially if we would implement online payments, the registration number isn't really needed much anymore, and it could be removed (or at least shown less prominently). And maybe it would help to show the waiting list position more prominentaly (i.e. where the registration number is currently, but we would need to explain it better, see #116).