Old review requests from years back didn't track the closing times of

review requests. This broke the new review request status information

code, which assumed a valid timestamp to parse and display. The old

mechanism ignored this due to templates in Django 1.6 silently ignoring

failures.
Now, if the timestamp for a close is None, we just won't attempt to

show timestamp information.

Unit tests pass.
Tested with review requests that were closed but lacking close info.