Gin adds sidebar to modals

Problem/Motivation

Gin adds the node edit form sidebar to modal dialogs.

Steps to reproduce

Open the “Change site assignment for this [content type]” modal.
An empty sidebar element is rendered at the bottom of the modal and appears cut off.

Proposed resolution

The modal form is now built with the _wrapper_format query parameter temporarily set to drupal_modal, so themes like Gin can correctly detect it as a modal request. This prevents Gin from adding node-edit-form-specific UI elements, such as the sidebar/meta fields, to the Sites selection modal.

Remaining tasks

User interface changes

API changes

Data model changes