Repair individual post collapse in Topic namespace

The code that disabled collapsing topics in the Topic namespace also
ended up stopping moderated posts from being collapsed.  Adjust the
code so it only prevents collapsing the topic.

(cherry-pick from 7c023f7b4d74cedea5ce0de1c038494c1f8c9cd1)
Bug: 71132
Change-Id: I133bfaf0ed22bb1e8d3ba7b0b461ca8f0d5de389
Merge "Repair individual post collapse in Topic namespace" into wmf/1.24wmf22