R1:a3c43c473be9

Convert dashboard read/display pathways to the new panel storage format

Summary:
Depends on D20406. Ref T13272. This gets about half of Dashboards working with the new "duplicate panel friendly" storage format. Followups will fix the write pathways.

Collateral damage here includes:

- Remove the old Dashboard/Panel edge type. We have a new, more general edge type for "container X uses panel Y", and we don't need this edge type for anything else.
- Remove "attachPanels()" from Dashboard. Only…