Proper support for required fields in payment provider settings

Closes #22

src/pretix/plugins/banktransfer/payment.py

@@ -19,7 +19,6 @@ class BankTransfer(BasePaymentProvider):
                  forms.CharField(
                      widget=forms.Textarea,
                      label=_('Bank account details'),
-                     required=False
                  ))
             ]
         )

src/pretix/plugins/paypal/payment.py

@@ -35,17 +35,14 @@ class Paypal(BasePaymentProvider):
                          ('live', 'Live'),
                          ('sandbox', 'Sandbox'),
                      ),
-                     required=False
                  )),
                 ('client_id',
                  forms.CharField(
                      label=_('Client ID'),
-                     required=False
                  )),
                 ('secret',
                  forms.CharField(
                      label=_('Secret'),
-                     required=False
                  ))
             ]
         )

src/pretix/plugins/stripe/payment.py

@@ -23,12 +23,10 @@ class Stripe(BasePaymentProvider):
                 ('secret_key',
                  forms.CharField(
                      label=_('Secret key'),
-                     required=False
                  )),
                 ('publishable_key',
                  forms.CharField(
                      label=_('Publishable key'),
-                     required=False
                  ))
             ]
         )