Correct an issue where "dashboard.panel.edit" fatals in the API console

Summary:
Ref T13650. Currently, viewing the API console help page for this method fatals because it constructs a generic, untyped panel.

As a step toward improving this, generate a concrete panel type instead. This isn't the best possible fix, see T13650 for discussion.

Test Plan: Viewed "dashboard.panel.edit" API page, now saw a usable page.

Maniphest Tasks: T13650

Differential Revision: https://secure.phabricator.com/D21668