Optimize the screenshot, file attachment, and no-repository code paths.

The screenshot page was quite slow still, as it didn't take advantage of
the new caching abilities in the models. We now have the query count
down pretty extensively.

File attachments on the review request page were pretty costly too, due
to a special template tag we have that serializes all the file attachment
comments. The problem there was that we weren't accessing the right
caches. Primarily, they were accessing comments directly. FileAttachment
and Screenshot both have get_comments() functions that both return from
the cache and sets it, which solves this.

Review requests without repositories were still trying to compute diffs,
which was a waste of effort. This happened on the review request page,
screenshot page, and on the Last Updated check. Now we skip these if
there are no repositories. This skipping is done in a new convenience
method, ReviewRequest.get_diffsets.

The query counts on the screenshot page and for review requests
with file attachments went down to the same levels as standard review
request pages.

Review requests with no repository decreased in size a bit.