Allow dashboard panel policies to be edited

Summary: Fixes T4982, expose dashboard panel policy editing to UI

Test Plan: Create panel, verify that user can edit who can see and who can edit panel

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4982

Differential Revision: https://secure.phabricator.com/D9238