LEARNER-7136 populate expiry date and add mechanism for new verification

Method approve() is overridden to accommodate new users that will have their verification approved in the future.
The approve() method is called from software secure results callback every time a Software Secure Photo Verification
is approved.The method overridden now updates expiry_date and then calls super to perform task it was performing
before this change.The expiry_date set is the same as the one sent in verification approval email.

In results_callback for sspv the expiry_date and expiry_email_date for the most recent previous verification if exists
are also updated to NULL.