Fix up topic reply link.

When clicking 'reply' on a topic when the topic is collapsed should
open the discussion and take the user to the reply form.

1. Make it consistently use the activateForm handler.
2. In the activateForm handler, if the form is not visible,
   check collapse state and toggle inversion for that topic if
   it's helpful.

Change-Id: I07c0e9d78c8ad8e2ceb6458a7b7add7a20aa98c6