assignment: Fix period ends in email.

www/conservancy/apps/assignment/forms.py

@@ -65,3 +65,5 @@ class AssignmentForm(forms.ModelForm):
 
         if self.cleaned_data['period_end_type'] == 'a specific past date' and not self.cleaned_data['period_ends']:
             raise ValidationError('This field is required')
+
+        return self.cleaned_data['period_ends']

www/conservancy/apps/assignment/views.py

@@ -22,7 +22,7 @@ class AssignmentCreateView(CreateView):
             ['denver@sfconservancy.org', 'bsturmfels@sfconservancy.org'],
         )
         return super().form_valid(form)
-    
+
     def get_success_url(self, *args, **kwargs):
         return reverse_lazy('assignment-thanks', kwargs={'pk': str(self.object.uuid)})