Implement ticket download dates (closes #15)

src/pretix/base/forms.py

@@ -81,8 +81,7 @@ class SettingsForm(forms.Form):
 
     def __init__(self, *args, **kwargs):
         self.obj = kwargs.pop('obj')
-        if 'initial' not in kwargs:
-            kwargs['initial'] = self.obj.settings
+        kwargs['initial'] = self.obj.settings
         super().__init__(*args, **kwargs)
 
     def save(self):

src/pretix/base/settings.py

@@ -73,6 +73,10 @@ DEFAULTS = {
         'default': 'True',
         'type': bool
     },
+    'ticket_download_date': {
+        'default': None,
+        'type': datetime
+    },
     'last_order_modification_date': {
         'default': None,
         'type': datetime