Add "tabindex" to Remarkup reference lists

Summary:
Prevent keyboard focus of these links so we don't disrupt tab order from
comments to "Submit".

Arguably I should make a "function" for this or something but there's nowhere to
really put it that makes any sense right now.

Test Plan: Verified Firefox skips these links in tab order.

Reviewers: fratrik, btrahan, jungejason

Reviewed By: fratrik

CC: aran, fratrik

Maniphest Tasks: T661

Differential Revision: 1180