Fix an error on the review_detail page about an undefined variable.

The recent collapsed reviews change could cause an undefined variable
error, due to the incorrect assumption that the variable would be defined
later. On review requests that have updates but no reviews, this wouldn't
be the case. The reason for this is that we would declare this variable,
last_timestamp, per review, instead of outside the reviews loop. This would
also cause excess SQL statements.

Now we declare it once before we loop over anything.

This also consolidates some of the logic. We had nested ifs when they should
themselves have been collapsed.

Tested with my review request that had only change descriptions. It no longer
crashed, and collapsing worked fine.