fix for link to authored TOP_LEVEL_CONTAINERs
fix for link to authored TOP_LEVEL_CONTAINERs

Without this change when logged in as the default teacher 
and viewing one of my classes at this url:
  http://localhost:3000/portal/classes/1

The link to "Activities" at the top looked like this:
  http://localhost:3000/portal/external_activities

Which is incorrect -- and I'm not sure why.

concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse,concord-consortium/rigse