Implement set pricing options at global_setting (fossasia#418)

* Implement set pricing options at global_setting

* move ticket_fee_percentage

* Update code

* Update code

src/pretix/control/forms/global_settings.py

@@ -2,6 +2,7 @@
 
 from django import forms
 from django.conf import settings
+from django.core.validators import MinValueValidator
 from django.utils.translation import gettext_lazy as _
 from i18nfield.forms import I18nFormField, I18nTextarea, I18nTextInput
 
@@ -145,6 +146,19 @@ def __init__(self, *args, **kwargs):
                 label=_('Stripe Webhook: Secret key'),
                 required=False,
             )),
+            (
+                "ticket_fee_percentage",
+                forms.DecimalField(
+                    label=_("Ticket fee percentage"),
+                    required=False,
+                    decimal_places=2,
+                    max_digits=10,
+                    help_text=_(
+                        "A percentage fee will be charged for each ticket sold."
+                    ),
+                    validators=[MinValueValidator(0)],
+                ),
+            )
         ])