MIT CCX: Prevent the discussion tab from displaying

When the POC feature is enabled and there is an active poc displaying discussions leads to undefined behaviors.  Hide the tab to prevent usability problems.

common/lib/xmodule/xmodule/tabs.py

@@ -376,6 +376,10 @@ class DiscussionTab(EnrolledOrStaffTab):
         )
 
     def can_display(self, course, settings, is_user_authenticated, is_user_staff, is_user_enrolled):
+        if settings.FEATURES.get('PERSONAL_ONLINE_COURSES', False):
+            from pocs.overrides import get_current_poc
+            if get_current_poc():
+                return False
         super_can_display = super(DiscussionTab, self).can_display(
             course, settings, is_user_authenticated, is_user_staff, is_user_enrolled
         )