Prevent useless comment fetching for screenshots and file attachments.

Screenshots and file attachments without comments weren't having their
_comments variable set, which was causing us to do a fetch later on
for each screenshot and file attachment. We now make sure to always
set _comments in review_detail, preventing these extra, useless fetches.

Tested on some review requests with many active and inactive screenshots
and file attachments. Saw that the extra queries went away and page load
time decreased.