[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
- [x] Maybe add more tests
- [ ] 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.
I fix it a little bit different. I did not added mailto links, because I want make different look between user that have profile and link to it and user that do not have profile.
Please take a look I guess you will like it.