The "Posted Time" should be the time of publish.

Currently, the time_added of a review request is setup when a new record is
added. If there is a time difference between the creation time and the time of
actually publishing the review request, the field is not updated and it shows
the time of creation as the "Posted" time. This is fixed by updating the
"time_added" field to current time when a review request is published.

Testing done: Manual, Ran all tests to ensure no other breakage.

Reviewed at http://reviews.reviewboard.org/r/3753/
Fixes bug 1897.