When registeration is complete, the RTS needs the sid, which was previously only sent to the HS. This update will also store it in the signupInstance so that it can be sent to the RTS.
This will happen anyway when they follow email verification links.
make captchas poll for success so if they are completed elsewhere, electron moves on
Generate a client secret in the Signup class (if we don't already
have one) and re-usae it for subsequent attempts to register,
that way the IS can honour the sendAttempt flag and not re-send
the email if we're just retrying and requestToken becomes
idempotent.
Use the new register-specific request token endpoint (https://github.com/matrix-org/matrix-js-sdk/pull/147) and catch the error that it gives if the email is already in use. Also add initial values to the registration form so we can reload it after the error without all the values disappearing, and split out the guest username parameter which was previously called defaultUsername.
Recaptcha is a special snowflake because it dynamically loads the script
and THEN renders with info from the registration request. This means we
need a back-channel for the UI component to 'tell' the stage that everything
is loaded. This Just Works which is nice.
