Fix #531 -- Make placeholders replace in subject (#594)

Placeholders in subject were not being replaced because there was
no `.format()` called on the subject.

This commit creates a context dict that is used for both the body
and the subject. It is then replaced using `.format_map()`

Fixes https://github.com/pretix/pretix/issues/531

src/pretix/plugins/sendmail/forms.py

@@ -23,7 +23,11 @@ class MailForm(forms.Form):
         super().__init__(*args, **kwargs)
         self.fields['subject'] = I18nFormField(
             widget=I18nTextInput, required=True,
-            locales=event.settings.get('locales')
+            locales=event.settings.get('locales'),
+            help_text=_("Available placeholders: {expire_date}, {event}, {code}, {date}, {url}, "
+                        "{invoice_name}, {invoice_company}"),
+            validators=[PlaceholderValidator(['{expire_date}', '{event}', '{code}', '{date}', '{url}',
+                                              '{invoice_name}', '{invoice_company}'])]
         )
         self.fields['message'] = I18nFormField(
             widget=I18nTextarea, required=True,