Decouple Classy libraries from thunder_admin

Problem/Motivation

As stated in #3050389: [META] Remove dependency to Classy from core themes, Classy will be moved to contrib before Drupal 9 and Seven has been decoupled from classy in #3107872: Decouple Classy libraries from Seven.
Since Thunder_admin uses Seven as a base theme, it has to align to these changes.

Proposed resolution

Find references to Classy (libraries overrides and imports).

Use Seven's replacement libraries and assets, move to thunder_admin if necessary