Fix regression in comment dialog dragging.
The way that the comment dialog was made draggable regressed, possibly due to

upgrading jQuery UI. The result was that the entire dialog except for the

resize grips was treated as draggable, making it impossible to use the mouse to

select text in the editor. Switching it to pass in a selector instead of a

jQuery makes it work properly again.

Opened a comment dialog on a diff and checked that I got the correct cursor on

the title of the dialog, and that I could only drag that dialog around by

clicking on the title. Checked that I could now select text in the editor.