Make the default course preview page less sketchy

@sefk, I guess?

Requested user to test and confirm fix, since we don't have test data for Instructor. Given the lack of Instructor objects in our prod db and the breakage, I was tempted to declare Instructor dead and submit a patch ripping it out and default preview pages along with it, replaced by instructions asking people to just make a file like course_handle.html.  But I couldn't tell if that was a good idea or if I was seduced by my late-evening grumpiness. So I decided to push this much less radical edit instead. 

* Add comments to the template explaining that it's sketchy
* Add tests to the template so it looks a little intentional when it's
  not filled in.
* Patch to instructor model to fall back to local storage. Seems difficult to
  test.
* (closes #2487)