Fix an issue where editing a dashboard panel from a dashboard could duplicate the panel

Summary:
Depends on D20472. Ref T13272. Currently, when you edit a panel from a dashboard, we try to add the panel to the dashboard. This always works since dashboards no longer enforce panel uniqueness, and you can end up with duplicate panels.

Instead, only add panels if we're creating them.

Test Plan:

• Edited an existing panel, no duplication.
• Created a new panel, saw it added to the dashboard.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20473