Legalpad - remarkup LX to link to LX

Summary: standard ish. Fixes T4388.

Test Plan: made a comment with L1 and noted L1 linked to L1. Also observed working-ish hovercard.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4388

Differential Revision: https://secure.phabricator.com/D8178