Refactor collapsing to handle moderated posts

* Get rid of flow-topic-collapsed-invert
* Use flow-topic-expanded and flow-topic-collapsed
* Save + (expanded) or - (collapsed) to storage, since there is no
  general toggle class any more
* Remove flow-topic-collapsed when explicitly overriden, either by a
  user click or session storage (essentially a past click)
* Don't remove collapsed class of moderated posts when switching the
  board's overall collapse mode
* Add browser tests for the above

See https://trello.com/c/ZR3a5BAF/282-b-171-issues-with-closed-topics-in-collapsed-views

Change-Id: I1d8274d7306bc318f693a6a10b712e56219ea25c