Re-order flows so that email auth is done last
Re-order flows so that email auth is done last

It's more natural for the user if the bit that takes them away
from the registration flow comes last. Adding the dummy stage allows
us to do the stages in this order without the ambiguity.

matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse,matrix-org/synapse