On Dashboards, distinguish between invalid panels and restricted panels

Summary:
Depends on D20374. Panels may not be visible if they are restricted (no permission) or if they are invalid (e.g., the panel was deleted).

Render these as two separate states instead of one big combined state.

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20376