[Form] Add a 'submitted' attribute to the form view

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#3029

Basically, the use case of this PR is to determine in the view if a form has been validated by the validator. By default, the valid attribute is true and we can't know if the form has been submitted. Now, we can use `form.vars.submitted && form.vars.valid`.