Post Cloaking:

* We now use a new custom view, {{cloaked-collection}} to display posts in a topic.

* Posts are removed and inserted (cloaked/uncloaked) into the DOM dynamically based on whether they
  are visible in the current browser viewport.

* There's been a lot of refactoring to ensure the relationship between the post views and the topic
  controller is sane.

* Lots of fixes involving jumping to a post, including a new LockOn component to that tries to stay
  focused on an element even if stuff is loading before it in the DOM that would normally push it
  down.