Delegate forms $data validation to Forms::submit

[Raruto]

Preamble

This pull request doesn't alter current forms submission behavior (in essence it ensures that the "forms.submit.before" event is always triggered for each form submission).

A borderline example is shown below.

---

Issue description:

When using a form call like the following:

<form action="/api/forms/submit/contact" method="post" enctype="multipart/form-data">
  <input name="files[]" type="file">
  <input name="submit" type="submit" value="Submit">
</form>

empty $_POST data is passed to Forms\Controller\RestApi::submit causing an early exit from that function and without being so able to parse $_FILES data inside the "forms.submit.before" hook

https://github.com/agentejo/cockpit/blob/efb8944051d7f347a4114cd21d8f01ecab373a00/modules/Forms/Controller/RestApi.php#L23-L34

Proposed solution:

Set fallback param $data to empty array and delegate empty check validation to Forms::submit function.

// Forms\Controller\RestApi::submit

$data    = $this->param('form', []);
$options = $this->param('form_options', []);

return $this->module('forms')->submit($form, $data, $options);

// Forms::submit

$this->app->trigger('forms.submit.before', [$form, &$data, $frm, &$options]); // <-- parse here form $data request

if (empty($data)) {
  return false;
}

---

Example of usage:

Dynamically check and populate forms $data['files'] entry:

// config/bootstrap.php

$app->on('forms.submit.before', function($form, &$data, $frm, &$options) use ($app) {

  // see "Helper Functions" for more info about it
  $files = get_uploaded_files()

  if (!empty($files)) {
    $files = $app->module('cockpit')->uploadAssets('files', ['folder' => get_forms_uploads_folder()]);
    $data['files'] = $files['uploaded']; // <-- save entries as filename
  }

});

Contact Form

Simple contact form template with **single input `files[]`**:  **Lexy template:** ```blade @form( 'contact', [ 'id' => 'contact-form', 'class'=>'contact-form' ] )

@lang('Contact us'):

@lang('Name') *

@lang('E-mail') *

@lang('Phone')

@lang('Message') *

@lang('File')

{{ $app("i18n")->getstr('I accept the privacy policy and I give consent to processing of this data as established by the GDPR', [ $app['base_route'] . '/privacy-policy/', 'https://gdpr.eu/' ] ); }} *

@lang('Thank You! I\'ll get back to you real soon...')

@endform ```

Upload Form

Simple contact form template with **multiple input `files[]`** :  **Lexy template:** ```blade @form( 'upload', [ 'id' => 'upload-form', 'class'=>'upload-form' ] )

@lang('Upload some files'):