Restrict indentation of edit forms and previews.

Previously, replying to a second-level reply meant that the reply
would be indented (by virtue of being a reply), and then inside that
the edit form would be indented (because edit forms are indented),
and the preview would be indented (because it has the flow-post
class, and flow-posts inside other flow-posts are indented).

Now, edit forms are indented on top-level posts only, previews are
no longer indented, and reply forms are indented as previously.

Change-Id: Ic14395738759d74d69ef0665bb24a5278711378a