Non-exclusive offers that only allow combination with specific other offers

* enforce max_affected_items during offer application

also restrict the usage of offer consumption to line.consume()

* improve test coverage

* flesh-out initial opaque data-model

refactors the basket line consumption into a facade object.

the goals is accomodating combitions of various offers on the same basket-lines,
while remaining backward compatible with the current single offer per line
policy.

* add docstrings

* add exclusive flag to conditionaloffer model and continue refactoring

* test new offer consumption on basket view

* cleanups

* fix exclusivity

* code style

* add cast to pacify tests

* spelling

* add some documentation to the release notes

* move documentation to new 1.6 release docs