Allow filtering by active

* add in a MktFilterBackend which munges 0 into True and False so that we can use nicer non-Python values
this seems overly magical. Why not apply use the serializer's boolean field and apply the result of the active field to [the queryset](https://github.com/andymckay/zamboni/blob/1066356/mkt/inapp/views.py#L48)?