Improve detecting already signed comments
Previously, we required a signature at the end of the comment.
This was a pretty rough heuristic that did not correctly handle
many comments that we would consider entirely properly signed
in CommentParser (e.g. comments wrapped in formatting like
<small>…</small>, comments with a post-scriptum or in parentheses,
or comments generated by various templates).
Now we process the user input using the same code that adds reply
links, and only add a signature when we detect that there really
isn't a signature (including template-generated), or if the signature
is in the wrong place and would result in the reply link showing up
in the wrong place as well (not at the end of the comment).