Admin theme for modal forms

[rlnorthcutt]

When using the modal forms for LB and on the front end, we get the front end theme by default. Since we are moving to Claro in the near future, and since we use the admin theme for content editing, it makes sense to have the admin theme used when we are editing content/entities inside a modal.

Additionally, anytime we have entity browser in a modal, it uses the admin theme... so this would help provide more consistency across the board.

[rlnorthcutt]

Standard: always use the admin theme when creating or editing "content". That experience should be uniform regardless of where a user does it.

[kevinfunk]

Layout Builder Modal has an issue for this https://www.drupal.org/project/layout_builder_modal/issues/3042907. With the the release of 1.0-alpha2, we should be able implement a custom Layout Builder Modal theme.

[kevinfunk]

Related issue for Media Library https://www.drupal.org/project/drupal/issues/3050508