Make user registration fields configurable

[flegfleg]

right now the Telephone & Address are mandatory, hard-coded fields.

two possibilities:

1. make current system configurable (huge effort)
2. remove the additional fields from cb plugin (but keep the fields available for existing users). Recommend external plugin for new installations.

i feel strongly 2. is the way to go.

[schrandy]

2. when it will still be mandatory to add address before booking.