set invoice_number at contribution->create, not invoice creation

Last month https://github.com/civicrm/civicrm-core/commit/9cad3ff4174de4bcad980d5bac6038735340e582 created new searchable field for human readable invoice number on contributions. The field is currently only populated when an invoice is created for emailing or printing or downloading from the User Dashboard, which leads to problems when using the field on Find Contributions or Advanced Search as it is not populated for contributions until one of these actions is taken.

Let's change when the field is to time of contribution creation, and remove setting it at time of invoice creation. 

The best practices around invoice numbers, set in law in some places like the UK, I believe, is to not change the invoice number after it has been created. The previous implementation had an undocumented feature  that caused them to be changed when emailing or printing or downloading from the User Dashboard if a change had been made in the invoice prefix on the CiviContribute Component Settings since last time invoice was printed or emailed or downloaded.