…rm (Ener-Getick)

This PR was merged into the 3.2-dev branch.

Discussion
----------

Deprecate using Form::isValid() with an unsubmitted form

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #7737
| License       | MIT
| Doc PR        | not yet

> I'm calling out for you to decide how to resolve the inconsistency between the Form::isValid() method and the ``valid`` variable available in the template:
>
> - ``isValid()`` returns ``false`` if a form was not submitted. This way it is possible to write concise controller code:
>
> ```php
> $form = $this->createForm(...);
> $form->handleRequest($request);
> if ($form->isValid()) {
>     // only executed if the form is submitted AND valid
> }
> ```
>
> - ``valid`` contains ``true`` if a form was not submitted. This way it is possible to rely on this variable for error styling of a form.
>
> ```twig
> <div{% if not form.vars.valid %} class="error"{% endif %}>
> ```
>
> We have two alternatives for resolving this problem:
>
> 1. Leave the inconsistency as is.
> 2. Make ``isValid()`` return ``true`` if a form was not submitted (consistent with ``valid``)
> 3. Revert to the 2.2 behavior of throwing an exception if ``isValid()`` is called on a non-submitted form (not consistent with ``valid``).
>
> Both 2. and 3. will require additional code in the controller:
>
> ```php
> $form = $this->createForm(...);
> $form->handleRequest($request);
> if ($form->isSubmitted() && $form->isValid()) {
>     // only executed if the form is submitted AND valid
> }
> ```
>
> What do you think?

This PR implements the option 3 as it was the most chosen in #7737

Commits
-------

2c3a7cc Deprecate using Form::isValid() with an unsubmitted form

see symfony/symfony#17644