Invoices: file paths were using the old system - updated to Contao 3 DBFS

Only the ID of the file is saved in the database now, not the complete
absolute path.
Also made a small change in alias generation (otherwise temporary
aliases like -1, -2, -3 etc. are generated under certain circumstances)
consider useAutoItem setting in InvoiceList

Contao 3 has a setting that allows you to omit the 'items/' part of the
URL. This has to be considered when generating the item URL (idk if
there is a better way to do it)