Issue #3611403: Destroy Dropzone instance on detach to avoid duplicates after...

velmir_takyrequested to merge
issue/dropzonejs-3611403:3611403-duplicate-dropzone-instances into 8.x-2.x

Issue #3611403: Destroy Dropzone instance on detach to avoid duplicates after AJAX rebuild; add functional JS test

Closes #3611403

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