Allow dashboard panels to be archived
Summary: Ref T5471. Adds an archived state for panels. Archived panels don't show up in the default query view or in the "Add Existing Panel" workflow.
Test Plan:
- Archived a panel.
- Activated a panel.
- Viewed / searched for archived/active panels.
- Popped "Add Existing Panel" dropdown and saw it omit archived panels.
Reviewers: chad
Reviewed By: chad
Subscribers: epriestley
Maniphest Tasks: T5471
Differential Revision: https://secure.phabricator.com/D9779