[WIP] Merge Cart and Order models

This PR refers to: https://github.com/Sylius/SyliusSalesBundle/issues/30 The aim is to delete the duplication between Cart and Sales bundle.

*This PR depends on 2 others:*
* First step is updating Order model to be able to deal with Cart system. This is done by https://github.com/Sylius/SyliusSalesBundle/pull/31
* Second step is updating CartBundle to use Order model. This is done by https://github.com/Sylius/SyliusCartBundle/pull/37

Third step is using updating CoreBundle. This is what this PR does.

Todo:
- [x] Update from Cart to Order
- [x] Delete unused Cart model
- [x] Re-use Cart models, that now extend Order ones
- [x] Simplify *a lot* FinalizeStep