Add a DUMMY stage to captcha-only registration flow

This allows the client to complete the email last which is more
natual for the user. Without this stage, if the client would
complete the recaptcha (and terms, if enabled) stages and then the
registration request would complete because you've now completed a
flow, even if you were intending to complete the flow that's the
same except has email auth at the end.

Adding a dummy auth stage to the recaptcha-only flow means it's
always unambiguous which flow the client was trying to complete.
Longer term we should think about changing the protocol so the
client explicitly says which flow it's trying to complete.

vector-im/riot-web#9586