prevent voucher from being set to a duration with negative hours (#502)

smash/web/forms/voucher_forms.py

@@ -76,6 +76,8 @@ class VoucherForm(ModelForm):
         self.fields['issue_date'].widget.attrs.update({'pattern': '[0-9]{4}-[0-9]{2}-[0-9]{2}'})
         self.fields['issue_date'].widget.attrs.update({'required': False})
         self.fields['issue_date'].initial = timezone.now()
+        #hours
+        self.fields['hours'].widget.attrs.update({'min': 0})
         # expiry date
         self.fields['expiry_date'].widget.attrs['readonly'] = True
         self.fields['expiry_date'].required = False