Invoice filtering for normal users

* open_invoices.php: Let regular users see only their own invoices
(additional if (sesAdminAccess()) query in $strQuery)

* list_switch.php: Let regular users see only their own invoices
(list/archived) by adding filter to $strFilter

* invoice_printer_contactperson.php: Changed $intStartY (bottom form
offset) from 179 to 175