improve pretix voucher creation workflow

[marcoacierno]

1. Rename the model from SpeakerVoucher to ConferenceVoucher
2. Automatically create the voucher on pretix? Sync if the voucher is deleted / used / similar from pretix
3. Add new types "Staff" / "Grant" to ConferenceVoucher
4. Change how the voucher is created: In https://github.com/pythonitalia/pycon/blob/59b706834fccb4ac5bf0b1d70d9957b97220b38f/backend/conferences/admin.py#L482 we should create the vouchers, outside the request-response cycle