https://github.com/matrix-org/synapse/issues/7512 means that (at least)
sometimes after clicking on the email validation link and being redirected
to riot, the server will claim the email identity auth stage is still incomplete.
This meant that we displayed the email identity UIA component but with an empty
email address, because we don't know that in the new session. Work around this by
assuming that if the email UIA component is being displayed but we don't have an
email address input, the link has been clicked and we're just waiting for the poll.
Also don't fire off an initial register request if we're already mid-UI-auth, because
that's confusing and unnecessary.
Also also remove unused requestingToken state.
Fixes https://github.com/vector-im/riot-web/issues/13434
523067e5f9