Normalize Phriction links when looking them up in remarkup

Summary: Fixes T10845.

Test Plan: Verified that [[ quack ]] and [[ QUACK ]] both work. Previously, the link had to exactly match the capitalization of the target.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10845

Differential Revision: https://secure.phabricator.com/D15777