Fix duplicate "Markdown Reference" links.

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.

Reviewed at https://reviews.reviewboard.org/r/5749/