Re-implementing lesson lookback

[nhurtig]

In the update to the progression system, a feature that was lost was the ability for users to click on their past lesson sets and revisit them. Under the current system, it's now possible to add it back in, but it would take a lot of work. The approach below isn't perfect because it doesn't play well with alternate lessons, but it's all I can think of with the current progression system.

• The current tutor view only takes an assignment ID. It would need to be changed to take the assignment ID as well as optional info about the requested state (set index, lesson index).
• There would have to be steps taken to differentiate between submissions made on past lessons and submission made at the current lesson, or else users' current states would be affected by them reviewing lessons.
• The frontend and URL routing would have to be changed to re-enable the links and route those links to the correct view.
• Alternate lesson activation would likely have to be disabled on the past lessons because there's no storage set up for keeping track of a user's state. Instead, a user would only be able to progress through the primary lessons and alternates wouldn't be activated (If alternates were allowed, there'd be no way to figure out where to put the user after they complete the alternate without persistent progress storage)