Fix email registration: pt. 1
Fix email registration: pt. 1

We look to see if there's already a user logged in and if there is,
restore that session instead of logging the user in as their new
account. We still set this 'is_registered' flag though, even though
in that case it's not a newly registered account that's being restored,
so don't set in that case.

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk