[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        |  -

- [ ] Submit changes to the documentation
- [ ] Maybe add more tests
- [ ] Fix tests

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.
I didn't found any name selectors in existing scenarios, so I have decided to incapsulate it into context to keep scenario text readable. I can't use selector by label name, because there are two fields with same "Email" label. 
I will fix it. I agree that in this exceptional situation it we can use input name.