Fix #634 -- Do not allow deleting the last date of an event series (#675)

* Checking for the last date in the event series before deleting a date. Last date in a event series should never be delted.

* Adding check to ensure that last date in a event series is not deleted. Editing unit test around deleting subevent to assert on alert-danger

* Increasing the scope of test_delete. We are now creating 2 subevents and testing deleting one and ensuring that the last one is not deleted

* Fixing alert text. Removing a redundant if condition for checking subevent count

* Adding assert for second event to ensure its not deleted

* Minor fixes and rebase

src/pretix/base/models/event.py

@@ -639,6 +639,9 @@ class SubEvent(EventMixin, LoggedModel):
         data.update({v.property.name: v.value for v in self.meta_values.select_related('property').all()})
         return data
 
+    def allow_delete(self):
+        return self.event.subevents.count() > 1
+
     def delete(self, *args, **kwargs):
         super().delete(*args, **kwargs)
         if self.event: