[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.
Those are one of the oldest ones, I have in mind creating PR that fix this, but IMO new one should be named more correctly (note that suggested by my is just proposal).