Fix #103 -- Implement Stripe Connect (#836)

* ...

* Upgrade Stripe API client

* Implement account choice

* Add disconnect and fix tests

src/pretix/plugins/stripe/forms.py

@@ -0,0 +1,20 @@
+from django import forms
+from django.utils.translation import ugettext_lazy as _
+
+
+class StripeKeyValidator():
+    def __init__(self, prefix):
+        assert isinstance(prefix, str)
+        assert len(prefix) > 0
+        self._prefix = prefix
+
+    def __call__(self, value):
+        if not value.startswith(self._prefix):
+            raise forms.ValidationError(
+                _('The provided key "%(value)s" does not look valid. It should start with "%(prefix)s".'),
+                code='invalid-stripe-secret-key',
+                params={
+                    'value': value,
+                    'prefix': self._prefix,
+                },
+            )