Add a centralized comment dialog instance.

Screenshots and the diff viewer each had their own global comment dialog,
which was fine until more than one type of thing needed a comment dialog
on the same page. With file attachments, we now need a shared dialog.

Now, reviews.js defines gCommentDlg and initializes it on page load. Other
instances can now just make use of it.

Screenshot and diff commenting still work correctly.

Along with my upcoming file attachment changes, commenting on file attachments
works too.