Move HTML classes from template_preprocess_image_widget to a template

Problem/Motivation

HTML classes should be usually added in templates instead of preprocess functions. Also, Drupal core modules shouldn't include represantional HTML classes.

Proposed resolution

Move the HTML classes from the preprocess function to the template. The classes should be added in a template in Classy / Stable but not in the module.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet