Onboarding with contract signing, better appointment booking

During onboarding you can now sign the data processing agreement (AVV) electronically; the signed copy arrives by e-mail.

• Appointment booking: agents work in their own time zone and state slots correctly; if a booking fails, the agent says why.
• Billing: invoices can be downloaded from the payment history.
• Terminal: call times show in the correct time zone.
• Users without an agent no longer get stuck in an onboarding loop.