Allow Nuance items to provide curtain panels, link to imported tasks, parse comments

Summary:
Ref T10537.

• Let nuance items render custom curtain panels.
• Add a custom panel linking to the imported task, if one exists.
• Actually extract comments properly.

Test Plan:
Unit tests, plus:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10537

Differential Revision: https://secure.phabricator.com/D15537