Fix duplicate "Markdown Reference" links.
Review Request #5749 — Created April 29, 2014 and submitted
MarkdownEditor could end up creating duplicate "Markdown Reference"
links when opening the same editor more than once. An easy repro case is
to draft a reply, save, and open it again. Every time that is done,
there would be an extra link.We now check for the existence of a link before attempting to add one.
Went through the repro steps. I wasn't able to create any dup links.