Make some remarkup handling in Config cleaner, fixing {{other.option} links

Summary:
Depends on D19609. Ref T13189. At some point, we switched from RemarkupEngine to RemarkupView and lost this piece of hack-magic.

Restore the hack-magic. It's still hack-magic instead of a real rule, but things are at least cleaner than they were before.

Test Plan: Viewed auth.require-approval, etc. Saw references to other config options linked properly.

Reviewers: amckinley

Maniphest Tasks: T13189

Differential Revision: https://secure.phabricator.com/D19610