Add topic & post single revision view

We used to have a page to show a full reivion view of topic & post, along
with replies, author count, comment replies etc.  In the new frontend,
it doesn't seem easy to grab a specific revision for a topic.  In addition,
revison is historical data, it's not possible to keep track of the replies,
author count etc for that pariticular revision. It would make more sense
to show just the revision text.

This fixes both topic & post revision view error, I think we should merge
this now and improve it later.

Bug: 67378
Change-Id: I928373284d02793964d99b6a111267c347220985
Merge "Add topic & post single revision view"