3346: Tidy up sanitization of reply flag
3346: Tidy up sanitization of reply flag

Still not great, but...

Authors: 5a2b2fafb7d2c6d4f14d4d87ce05a72aae93452c@chrisholmes

alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend,alphagov/verify-frontend