Improve LMSReferenceItemWidget

Setting this to needs review.
Follow-ups to create:

1. Improve Drupal\lms\Plugin\ModalSubform\EntityForm::access()
2. Scrolling to elements after AJAXing
3. Views exposed form submit on the LMS entity selection views shouldn't be in the modal buttons area
4. Next minor phpstan and phpunit
5. Extend test coverage with creation of new entities and modal on top of a modal (creating a lesson and its activities)

Addressed all threads, thanks for reviewing!

Moving to fixed! We can address anything that comes up in follow-up issues.

Automatically closed - issue fixed for 2 weeks with no activity.