When merging contacts, also include linked (recurring) invoices.

The current situation is that the merged old contact has been deleted and that recurring invoice appears to be linked to the old contact, but when you click on the contact, it no longer exists. The invoice will then not be issued and must be linked to the new merged contact again. This is all without error messages to the customer (subaccount user). He will only find out when it has gone wrong. And then it takes a lot of manual actions to fix this.

Quick fix: Make a clear warning that future scheduled and recurring invoices are NOT included when merging contacts and that they must be manually linked again after merging so that sending the invoice does not fail.

See also ticket #4821277.