Microlesson content sourced from API, not JSON file.

* Split the mechanism to switch tasks into its own view, out of js/views/lessons/microlesson.js.
* Split out appropriate template code.
* Updates to topic model and collection.
* Lesson List and Microlesson now share the same topic collection, so you don't have to fetch it more than once.
* Navigating from Lesson List to a Microlesson no longer reloads the page, but triggers navigation through the router.