Support mentions from dashboard panels

Summary: Fixes T6394. See that task for a description. See T6403 for a proposed long-term fix.

Test Plan: Mentioned a task from a dashboard panel.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6394

Differential Revision: https://secure.phabricator.com/D10750