Invoices: Allow issuing invoices only to businesses

In situations where every invoice has a significant accounting cost and
consumers usually do not need invoices, this can save a lot of money or
effort.

src/pretix/control/forms/event.py

@@ -939,6 +939,7 @@ class InvoiceSettingsForm(EventSettingsValidationMixin, SettingsForm):
         'invoice_show_payments',
         'invoice_reissue_after_modify',
         'invoice_generate',
+        'invoice_generate_only_business',
         'invoice_period',
         'invoice_attendee_name',
         'invoice_event_location',