make LMS accordion and progress datetime display respect settings.TIME_ZONE

@chrisndodge  

We noticed in the LMS that though all times are stored as "aware", due dates, especially, were all still displayed in UTC.  This PR makes that a bit better (at least at Stanford) by coercing the due datetimes displayed in the accordion and progress page to settings.TIME_ZONE.  It also paves the way for an individual preference to set timezone.