Fix #2784: Send registration_expired emails properly

The LDAP user instance doesn't have the user's email since this
doesn't get put into LDAP until after they've verified it. This is
deliberate as it avoids the user getting sent (general) emails before
they have an account. Unfortunately the tests didn't take this into
consideration and thus failed to pick this up.

Fix it by also passing the address from the PendingUser instance.

Change-Id: Id87682368fe880303141a1c807766182c5f26d9d