Move Dashboard panel controllers into a "panel/" subdirectory

Summary:
Depends on D20368. Ref T13272. Dashboards have a lot of controllers, try to organize them a little better.

Note "EditController" vs "EditproController". Yikes.

Test Plan: Loaded dashboards. No code changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13272

Differential Revision: https://secure.phabricator.com/D20369