Decimal places inconsistency

[Pacu2]

decimal_places should be equal to decimal places of the default currency, instead of being harcoded similar is done in Saleor:

https://github.com/mirumee/saleor/blob/master/saleor/settings.py#L292 https://github.com/mirumee/saleor/blob/master/saleor/discount/models.py#L59-L61

[WhyNotHugo]

It should likely be configurable.

However, making it configurable means that migrations might be non-trivial to implement properly, so this is likely not an easy issue.