Require permissions against root post for post content

History lines for unmoderated posts within moderated topics
were being displayed in the history.  Admins want to delete
just the topic and not worry about individual post.

This patch runs an additional permission check whenever the content
of a revision is requested.

Change-Id: I1918b749f8f7715181827fd5897092ab85b89e84
Merge "Require permissions against root post for post content"