Rename invoices tool

[timiwahalahti]

Now that Camptix Invoices is turned on by default, it occurred to me that some organizers might think they can generate an invoice to be paid via this tool. When, in fact, the "Invoices" under tickets is just a place to store and generate invoices for already purchased tickets. All big batch purchases should be done using the sponsor invoices tool under budget.

I think we should probably rename both, but not sure what would be better names.