Allow users to "Edit a Copy" when trying to edit a panel they don't own

Summary:
Fixes T5167. When clicking "Edit" on a dashboard panel you don't own, the UI now allows you to make a copy instead.

As a bonus, fixes T5259.

Test Plan: See screenshots.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5259, T5167

Differential Revision: https://secure.phabricator.com/D9505