SPIKE/POC - CSR-137 multi step form

@mikejw 

Pull Request for POC so you can easily see changes.

### DO NOT MERGE
The other/reverse side has the same error, when linking `Detail` to `User` with `detail.user.id`...

```
Catchable Fatal Error: Argument 1 passed to TransformCore\\Bundle\\AppBundle\\Entity\\Detail::setUser() must be an instance of TransformCore\\Bundle\\AppBundle\\Entity\\User, array given, called in /Users/eddieaboujaoude/Development/transform/CSR-Fast-Stream-backend/vendor/symfony/symfony/src/Symfony/Component/PropertyAccess/PropertyAccessor.php on line 438 and defined
```