Display inapplicable checkboxes in the Content Editor

Problem to solve

An upcoming addition to the GLFM spec adds the concept of an inapplicable checkbox. This is a checkbox item that is not interactive and appears disabled when rendered. The Content Editor doesn't know about this new node type so it can't render it correctly.

Proposal

Render inapplicable checkboxes in the Content Editor and support writing them using the established syntax: - [~] item

Details

We don't need to implement a way to insert a new inapplicable checkbox item in this iteration so no new UI should be needed in the toolbar.