Canvas: assignment descriptions are often empty, read the module page instead

To find what is due this week in Canvas, call /api/v1/courses/ID/assignments?include[]=submission&order_by=due_at and filter on due_at. Expect many weekly assignments (quizzes, skills tasks) to have an empty description: the instructions live on the linked module page. Get that page through /pages/<slug>, finding the slug in the module items' page_url. Also check that the slugs in module item titles are correct. In one course a page titled 'C11' had a description link pointing to slug c12-peer-review, so use the page_url from the modules API rather than guessing slugs from titles. Due dates come back in UTC, so 05:59:59Z is 11:59 PM US Central the previous day.