Handle Deep Properties in Constraint Violation Responses

## Handle Deep Properties in Constraint Violation Responses

### Acceptance Criteria
1. `AbstractController::createConstraintViolationResponse` handles deep properties and displays them properly.

With constraint violation paths looking like this:
```
[2][foo]
[2][some_id]
[2][some_id]
[2][some_name]
[4][amount]
[4][foo]
[4][some_id]
[4][some_id]
[4][some_name]
```

Response body looks like this:
```JSON
{
    "errors": [
        null,
        null,
        {
            "foo": [
                "This field was not expected."
            ],
            "some_id": [
                "A validation message.",
                "Some other validation constraint error."
            ],
            "some_name": [
                "Some error message."
            ]
        },
        null,
        {
            "amount": [
                "This value should be of type integer."
            ],
            "foo": [
                "This field was not expected."
            ],
            "some_id": [
                "A validation message.",
                "Some other validation constraint error."
            ],
            "some_name": [
                "Some error message."
            ]
        }
    ]
}
```
### Tasks
- Extract object to perform conversion of constraint violations to arrays to be returned in a response.
- Implement an algorithm that converts the `[nested][0][property]` syntax of Symfony Validator into a properly nested array.

### Additional Notes
- Items that are just array elements (i.e. Have a numeric path like `[0]`) must be prefixed with a string, or upon conversion to JSON the numeric keys will be lost.
- Deep properties can exist if using the [All constraint](http://symfony.com/doc/current/reference/constraints/All.html) in Symfony Validator.
- **Note:** This change is not technically backwards-compatibility breaking. It will break tests (Controller tests will need to add `$this->injectMockValidationErrorFormatter($this->controller);` in `setUp`.) But that's it.