Embed dashboard panels in comments

Summary: Fixes T4983, Panel prefix 'W' should be recognized as a shortcut to a dashboard panel

Test Plan: Open any comment input, type '{W1}', or other existing panel, preview should embed that panel.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4983

Differential Revision: https://secure.phabricator.com/D9215