Do not show hidden course modules to students in exercises API

After the pull request #1090 CachedContent includes hidden modules too.
Students must not see hidden modules in the course exercise list API,
so they are now filtered out.

Fixes #1224

course/api/views.py

@@ -257,7 +257,8 @@ def __module_to_dict(self, module, **kwargs):
     def list(self, request, *args, **kwargs):
         modules = []
         for module in self.content.data['modules']:
-            modules.append(self.__module_to_dict(module, **kwargs))
+            if module['status'] != CourseModule.STATUS.HIDDEN:
+                modules.append(self.__module_to_dict(module, **kwargs))
         return Response({"count": len(modules), "next": None, "previous": None, 'results': modules})
 
     def retrieve(self, request, *args, **kwargs):