Correctly use jquery to modify links in chat messages.

The code was previously passing the message string provided by the user
directly to jQuery - which works okish if the first character is '<' since
it'll parse it as HTML, but the chat messages don't. As a result, it was
sometimes being parsed as a selector, failing, and raising an exception.

The fix is to put the chat message into a DOM node (have the browser parse
the HTML) before doing the jQuery operation to modify the link targets.

Fixes #3670