Addressing some PHP8 incompatibilities - Dashboard

Summary:
Using dashboard/panels and addressing PHP8 incompatibilities.

Refs T13588

Test Plan: Create a dashboard with multiple panels, including a tabbed panel. Update the tabbed panel to have multiple tabs of content. Try to have a tabbed panel contain itself.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21871