R1:a68b6cfe6541

Use real Dashboard Panels to render the default hard-coded homepage, not hacky fake panels

Summary:
Ref T13272. Currently, the hard-coded default homepage looks like a dashboard but is actually rendered completely manually.

This means that various panel rendering improvements we'd like to make (including better "Show More" behavior and better handling of overheated queries) won't work on the home page naturally: we'd have to make these changes twice, once for dashboards and once for the home page.