Multiple submissions of the same form on double-clicking "Select entities"

Re-rolling patch against latest dev.

The patch from #7 seems to cause an issue with switching tabs on the Entity Browser dialog. Clicking other tabs doesn't do anything at all.

Re-rolled for latest 1.x dev.

@grathbone, I have no issues with other Entity Browser tabs.

I seems that in previous patch I missed some files. Re-rolled once again.

Looks like after drupal core update this patch not work anymore. I have used patch from #10 and all was fine. Now on my site drupal core is version 8.4.8 and dropzonejs same as before (1.0-alpha8), as result - form submit absolutely not working.

Quick investigation shows that `onFormSubmitSingle` function runs 2 times immediately: on first one it set current values and then prevent for submit

    if (previousValues === currentValues) {
      event.preventDefault();
    }
    else {
      $form.attr('data-form-submit-single-last', currentValues);
    }

This issue can be related to jquery update in drupal core. anyway it's need some work here.

Adding 'context' selector where 'onFormSubmitSingle' is being called fixes issue mentioned in #11.

Incorrect transliteration has been causing test fails lately.
Adding fix of incorrect transliteration in DropzoneJsUploadControllerTest.php file.

Update patch for dropzonejs 2.0.0-alpha4

Update patch to include the js files again

Updated patch to include the js files for the latest dev version.

Quick Question/Thought:

Why are we creating our own version of onFormSubmitSingle, while we already have provision for "Drupal.behaviors.formSingleSubmit" provided by core/drupal.form

Merely adding following piece of code can do our work.

    $form['actions']['submit']['#attributes']['class'][] = 'singleSubmit';
    $form['#attached']['library'][] = 'core/drupal.form';

Updated patch from #19 so it could be used on D9.

Patch in #21 looks good for me.

Needs to be a merge request now. Not sure if the server side check is really needed, what's the benefit of that?