[Core] [WebBundle] Implemented Guest checkout

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #90
| License       | MIT
| Doc PR        |  (Sylius/Sylius-Docs#199)

- [x] Submit changes to the documentation
- [x] Maybe add more tests
- [x] Fix tests
- [ ] Make possibility to disable Guest checkout

I have created a possibility for Guest customers to checkout only using email. Order entity now always has information about customer's email(even if customer was registered). I have removed duplicated registration logic from Security step. Registration form still exists on Security step page as is, but registration request will be addressed to main registration controller. After successful registration customer will be redirected back to checkout.
No, I think it should stay on the order, that's what Spree does and it feels most natural. Simplifies searching by e-mail etc.