viewSource gets correct source for replies to PMs

`modules['commentPreview'].viewSource`, when attempting to view source for a PM/modmail message which is a reply, was grabbing the source for the first comment in that thread. Generally speaking, `viewSource` should check post IDs and replies against what the user asked for to find the correct comment.

http://www.reddit.com/r/RESissues/comments/1949rr/bug_found_an_obscure_and_weird_bug_regarding_the/