Work around email-spamming Riot bug

5d9546f9 introduced a change to synapse behaviour, in that failures in the
interactive-auth process would return the flows and params data as well as an
error code (as specced in https://github.com/matrix-org/matrix-doc/pull/397).

That change exposed a bug in Riot which would make it request a new validation
token (and send a new email) each time it got a 401 with a `flows` parameter
(see https://github.com/vector-im/vector-web/issues/2447 and the fix at
https://github.com/matrix-org/matrix-react-sdk/pull/510).

To preserve compatibility with broken versions of Riot, grandfather in the old
behaviour for the email validation stage.