Extract code to set isEmailReceipt in Contribution.completeOrder

Extract code to set isEmailReceipt in Contribution.completeOrder

This consolidates all the code in the completeOrder that determines whether or not to send an email receipt.

Note that
- testCompleteTransactionWithEmailReceiptInput tests that input is respected
- for events the event::sendMail function will not send unless it's configured for the event - which
explains our final default of TRUE (testCompleteTransactionWithParticipantRecord)
- testCompleteTransaction covers the back-office case which also defaults to true
- I figure the cost of reloading the value for contribution_recur is pretty cheap compared to the confusion of passing values around
- testpayLater covers contribution_page with is_email_receipt