Fix the current course instance redirection when the newest instance is not open yet

[markkuriekkinen]

The current course instance redirection (URL contains course without any course instance identifier): https://plus.cs.aalto.fi/o1

a-plus/course/views.py

Line 110 in 6a7704d

class LastInstanceView(CourseMixin, BaseRedirectView):

From @jsorva:

In a meeting some time ago, I promised to return to the issue of the
“current instance link” that points to the most recent version of a
course. So, here’s how I’d prefer like it to work:

1. While a course is running, the link points to the active instance.
   [For O1, this is in during the fall term.]

2. After a course is over [in December], the link continues to points to
   the most recent instance. This continues even after next year’s instance
   has been created in A+ but isn’t yet open to the public [typically early
   in the next calendar year]. And it even continues after the next
   instance is technically visible in some form but there’s only the front
   page and maybe some other materials, but the actual course contents
   aren’t visible yet [this is often the case during the summer].

3. Only when the actual course contents are visible [ideally some time
   in August, but might be September] does the link start pointing at the
   new instance.

To summarize, I’d like the link to always point to the newest instance
whose actual contents are actually visible; the mere existence of the
instance isn’t enough. (I’m not sure how close this wish is to the
current behavior.)

Internal ticket: https://rt.cs.aalto.fi/Ticket/Display.html?id=20168

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My first hunch is that this should be easy to accomplish by tweaking the latest course instance selection in the LastInstanceView.