Function insert_or_undelete_course_page_row 

async fn insert_or_undelete_course_page_row(
    conn: &mut PgConnection,
    course: &Course,
    page: &PageSnapshot,
) -> ModelResult<()>

Creates the snapshot’s page empty under its id, or brings back the deleted page with that id, so that update_page can write its content and exercises next. Writes no page history: the content save that follows does. Errors with PreconditionFailed if the page belongs to another course.