interactive-auth now handles requesting email tokens
interactive-auth now has a callback to request the email token which
it will call at the appropriate time (now at the start of the
auth process if the chosen flow contain an email auth stage).
This does make this a breaking change, although not sure this is
used outside of Riot. We could make it backwards compatible by
having an option for the new behaviour. It may not be worthwhile
though.
https://github.com/vector-im/riot-web/issues/9586
