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.