bootstrap’s invalid feedback is disabled by selectize #1784

I did:

• [ x] Search for if my issue has already been submitted
• [ x] Make sure I'm reporting something precise that needs to be fixed
• [ x] Give my issue a descriptive and concise title
• Create a minimal working example on JsFiddle or Codepen
  (or gave a link to a demo on the Selectize docs)
• [ x] Indicate precise steps to reproduce in numbers and the result,
  like below

Bootstrap (4) provides an adornment to required items by way of the invalid-feedback class when the item was not filled in or set. It does not work for selectized elements.

Steps to reproduce:

1. Create a selectized dropdown inside a form with some items to choose from
2. Give it the required attribute
3. Put in an enclosing div tag with itself and an invalid-feedback div tag (per bootstrap 4)
4. The form should be setup with class="needs-validation" novalidate
5. Do not select any items in the selectized dropdown
6. Attempt to submit the form

Expected result:
The invalid feedback should display and the form should not be accepted

Actual result:
The invalid feedback never even engages. Repeat the steps above without selectizing the select element and you do you get the invalid-feedback.