One-liner update to DRY up default site handling

figures.sites.default_site() is returning the Site object matching
`settings.SITE_ID` if we are running in single site mode and that is
exactly what is called in single site mode for
`get_site_for_course(course_id)`, so just making it that the source of
truth is the same code

figures/sites.py

@@ -134,7 +134,7 @@ def get_site_for_course(course_id):
             site = None
     else:
         # Operating in single site / standalone mode, return the default site
-        site = Site.objects.get(id=settings.SITE_ID)
+        site = default_site()
     return site