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/
Don't allow "Hide Closed" to bleed into dashboard datagrids.

The toggle for hiding closed review requests on the user and all review request
grids would set a profile flag, and then bled into the "All Outgoing" datagrid.
This change adds some plumbing to prevent this from happening.

Testing done:
Reproduced the bug, and then tried with this change. Saw that with the profile
flag set, the dashboard datagrids showed closed review requests, but the others
did not.

Fixes bug 3319.

Reviewed at https://reviews.reviewboard.org/r/5746/