Add flag to ticket type that allows not sending invoices to users

If a purchase contains only tickets with a ticket type having ths
"prevent_invoice" flag set, the invoice will not be send to the user
iff the purchase total is 0,00 EUR.
Add property to check for conditional invoice sending
Don't send an invoice if only tickets preventing sending are part of a purchase
Don't show a hint expecting an invoice if none will be send
Use nicely formatted email receiver for purchase confirmation mail
Merge pull request #175 from EuroPython/feature/no-invoice

Allow ticket types to conditionally prevent sending the invoice PDF to the buyer