Implement lesson opening time window (exams)

Problem/Motivation

This will need some additional things:

1. Course start link cache must be invalidated when course changes (already done) if time window data is stored on LMS reference field on course, and on lessons if that data is stored on a lesson. Cache must also have lifetime set to expire when the time window opens.
2. Course nav - I think no changes, the time window will be only applied for starting a lesson, then a user can finish within a specified time limit or whenever.

Remaining tasks

User interface changes

API changes

Data model changes