Function unlock_first_chapters_for_user 

pub async fn unlock_first_chapters_for_user(
    conn: &mut PgConnection,
    user_id: Uuid,
    course_id: Uuid,
) -> ModelResult<Vec<Uuid>>

Unlocks the first chapter(s) with exercises in the base module (order_number == 0) for a user. Also unlocks any chapters without exercises that come before the first chapter with exercises.