custom_check: Update FILES_WITH_LEGACY_SUBJECT set.

This commit adds 'zerver/lib/email_notifications.py'
to the FILES_WITH_LEGACY_SUBJECT set.

Because the file can have 'subject' in the email sense,
it should be exempted from the 'avoid subject as a var' lint rule.

tools/linter_lib/custom_check.py

@@ -21,6 +21,7 @@ FILES_WITH_LEGACY_SUBJECT = {
     # These use subject in the email sense, and will
     # probably always be exempt:
     "zerver/lib/email_mirror.py",
+    "zerver/lib/email_notifications.py",
     "zerver/lib/send_email.py",
     "zerver/tests/test_new_users.py",
     "zerver/tests/test_email_mirror.py",