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