Fix welcome user
https://github.com/matrix-org/matrix-react-sdk/pull/3101 meant we
don't get logged straight in after registering if using an email
address, but this was the point at which we made a chat with the
welcome user. Instead, set a flag in memory that we should try &
make a chat with the welcome user for that user ID if we get a
session for them.
Of course, if the user logs in on both tabs, this would mean each
would make a chat with the welcome user (although actually this
was a problem with the old code too). Check our m.direct to see if
we've started a chat with the welcome user before making one (which
also means we have to make sure the cached sync is up to date...
see comments).
