Fix ordering of conditional offers

Offers by default are sorted using Meta class ordering. However,
`get_offers` was doing two queries and combining them together.
Effecitvely bypass the ordering.

Test for ordering basket offers was written in the process not being
aware of standard ordering. I left it there as it can be useful test in
future (when somebody removes standard ordering).

Fixes #964
Merge pull request #964 from tangentlabs/issue/964/offer-priority

Respect priority of ConditionalOffers