complete_transaction api should default to domain email for from if none on contribution page

When a contribution is completed using the completetransaction api the email defaulting is as follows

1) contribution page from email
2) logged in user

The second of these reflects the fact that functionality from the IPN is shared with the backend contribution page. This is historic rather than sensible.

The contribution.completetransaction is most commonly used during IPN type transactions and the domain contact is a more sensible default than 'user 1' or 'cron-user'. I will alter the api to allow the from_email to be set as input but to default to domain contact