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/base/services/mail.py

@@ -82,6 +82,7 @@ def mail(email: str, subject: str, template: Union[str, LazyI18nString],
                     'invoice_company': ''
                 })
         body, body_md = render_mail(template, context)
+        subject = str(subject).format_map(context)
         sender = sender or (event.settings.get('mail_from') if event else settings.MAIL_FROM)
 
         subject = str(subject)