Move some representational classes in Media Library to Classy and others to Seven and/or Claro, as appropriate

Problem/Motivation

From comment #2:

There're a lot of classes being added in media_library_preprocess_media, Drupal\media_library\Form\AddFormBase, Drupal\media_library\Plugin\Field\FieldWidget\MediaLibraryWidget and media--media-library.html.twig.
The module markup is supposed to only include classes that are used for providing functionality.

Proposed resolution

Move all representational classes to Classy and assure that any representational markup is overridable at the theme level.

Remaining tasks

Review

User interface changes

None

API changes

Classes added in the Media Library module are moving to classy. Other themes will need to add their own representational classes or copy those from Classy.

Data model changes

none

Release notes snippet