Add access to historical revisions.

* Includes a refactoring of timestamp display (I could not get to it in
  post.html.php because it is abstracted away)
* Still needs API module, access to header revisions.
* Copy is not final.

Change-Id: I4255fb669f4884d984307ec7e8197dd0bb1b7bbe
Merge "Add access to historical revisions."