Fix missing app variable in mail subject (#11745)

documentation claims that you can use the %(app)s variable in password_reset and email_validation subjects, but if you do you end up with an error 500

Co-authored-by: br4nnigan <10244835+br4nnigan@users.noreply.github.com>

changelog.d/11710.bugfix

+Fix a bug introduced in Synapse v1.18.0 where password reset and address validation emails would not be sent if their subject was configured to use the 'app' template variable. Contributed by @br4nnigan.

changelog.d/11745.bugfix

+Fix a bug introduced in Synapse v1.18.0 where password reset and address validation emails would not be sent if their subject was configured to use the 'app' template variable. Contributed by @br4nnigan.

synapse/push/mailer.py

@@ -178,7 +178,7 @@ class Mailer:
         await self.send_email(
             email_address,
             self.email_subjects.email_validation
-            % {"server_name": self.hs.config.server.server_name},
+            % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )
 
@@ -209,7 +209,7 @@ class Mailer:
         await self.send_email(
             email_address,
             self.email_subjects.email_validation
-            % {"server_name": self.hs.config.server.server_name},
+            % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )