Make the default course preview page less sketchy

* 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. Difficult to
  test.
* (closes #2487)
Unused Imports; fewer snarky comments

* preview/default snarky comments removed.
* Cleaned up unused and dupe imports in course/previews/views
Merge pull request #2488 from Stanford-Online/jrbl-default_preview_less_sketchy

Make the default course preview page less sketchy