Allow sponsorship payment terms(due date) to be changed

[ssinger]

Allow the payment terms(days until payment to be paid) set to values other than 30.
Also allow a deadline to be set where payment is due at the deadline(or on the invoice date) after this time.

[mhagander]

I like the principle f this, but a couple of notes.

1. docs :)
2. wouldn't it be easier to just always store the number of days for the invoice in the db, with a default value of 30? That would remove a bunch of special-casing at runtime, I think?
3. I'm not sure about the terms used? To me, "payment deadline" would be the date that this patch calls "payment terms", and the other way around. Doesn't mean it's right but it felt backwards to me, so maybe see if we can find some other terms that make it more clear.
4. Do we intentionally want to allow deadlines both before and after the actual conference date?

[ssinger]

1. I've added a section to the sponsor docs
2. I've set a default of 30 days for the terms, it didn't remove that much runtime logic
3. I've replaced "payment deadline" with "paymentdueby" does that make things clearer?
4. Yes I think we should allow deadlines after the conference date if explicitly picked as a deadline. If someone wants to do this explicitly the system shouldn't stop them they might have reasons (prioritizing sponsors with $x day payment terms over cashflow and collection leverage). I've adjusted the invoicehandler to make sure this works