Fix missing app variable in mail subject for password resets (#15352)

* Update mailer.py

Fix `KeyError: 'app'`

* Create 15352.bugfix

Signed-off-by: Cyberes <cyberes@evulid.cc>

---------

Signed-off-by: Cyberes <cyberes@evulid.cc>

changelog.d/15352.bugfix

+Fix missing app variable in mail subject for password resets. Contributed by Cyberes.

synapse/push/mailer.py

@@ -149,7 +149,7 @@ class Mailer:
         await self.send_email(
             email_address,
             self.email_subjects.password_reset
-            % {"server_name": self.hs.config.server.server_name},
+            % {"server_name": self.hs.config.server.server_name, "app": self.app_name},
             template_vars,
         )