Fix the display of draft comments in review UIs.

Draft comments from other users were being shown in review UIs, since we
weren't filtering out based on whether the current user had the ability
to see the comment.

This affected the image review UI, and anything else using the default
get_serialized_comments function.

Created a draft comment, and then viewed the page in another browser
without logging in.

Before this change, I could see the comment. After the change, I could not.

Unit tests pass.