- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

Add support for managing large amounts of vouchers grouped in sets.

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

I was confused by what the test does, and had to ask @codeinthehole to
explain. Hopefully made it's intention a bit clearer now.

The clean method was raising a KeyError when the date fields were empty.