Media types cannot be created in the UI without JavaScript

This blocks #2988433: Automatically create and configure Media Library view and form displays, which is a Media Library stable blocker.

Thanks, @rodrigoaguilera! And for writing the test first, you rock :)

I found an alternative approach that will not involve saving the media type twice -- hopefully this satisfies all.

Looks good. Thanks!

1. +++ b/core/modules/media/src/MediaTypeForm.php
   @@ -304,6 +304,18 @@ function ($item) {
   +    // If the media source has not been chosen yet, turn the submit button into
   +    // a normal button which doesn't submit the form, but instead rebuilds it
   +    // with the media source's configuration form visible. Normally this would
   +    // be done during an AJAX request, but that requires JavaScript. Without
   +    // JavaScript, it's possible to submit the form without a media source,
   +    // which causes a fatal error because ::save() rightfully assumes that the
   +    // media source has been chosen, and that a source field has been set up.
   +    if (empty($this->getEntity()->get('source'))) {
   +      $actions['submit']['#type'] = 'button';
   +    }
   

   This looks like a great solution. I think we can improve the comment by making the sentences shorter and more about what occurs rather than what occurs without the fix. Something like:

       // If the media source has not been chosen yet, turn the submit button into
       // a button. This rebuilds the form with the media source's configuration
       // form visible instead of saving the media type. This allows users to
       // create a Media type without JavaScript enabled. With JavaScript enabled
       // this occurs during an AJAX request.
       // @see \Drupal\media\MediaTypeForm::ajaxHandlerData()
   

2. +++ b/core/modules/media/tests/src/Functional/MediaTypeCreationTest.php
   @@ -0,0 +1,42 @@
   + * Ensures that media UI works correctly.
   

   Tests the media UI without JavaScript.
   Just so nobody helpful converts this to a JS test.

Thanks! Fixed.

Sending this little tyke back to RTBC because all that changed in the last patch is comments.

Committed and pushed 3ba6bc1168 to 8.7.x and 897f0e461e to 8.6.x. Thanks!