Fix chainability of $.fn.*_infobox

When I consolidated the code for the infoboxes, I broke jquery chaining. This
wasn't immediately obvious because it was only used for the user infobox
created from a draft reply comment (the "Add Comment" handler). This change
fixes it.

Testing done:
I could "Add comment" again.

Reviewed at https://reviews.reviewboard.org/r/6184/