Fix: #189 on KA Lite central

Fixes https://github.com/fle-internal/ka-lite-central/issues/189#issuecomment-57759064

Basically request.is_admin doesn't handle the situation where a non staff central server admin creates a new student, so we were advancing to the else, which assumes a new student was signing themselves up, so our central server code was throwing a weird error. Thanks @mikewray for catching this!