R1:d0078570cc50

Convert dashboard panel storage to a format which can handle duplicates

Summary:
Ref T13272. See PHI945. Currently, dashboards tend to break when they have duplicate panels. Partly, this is because all the edit operations operate on a "panelPHID", so there's no way to say "remove the copy of panel X at the bottom of the right-hand column", since the operation is `remove(phid)` and that doesn't point at a specific copy of that panel.

In theory, the code is supposed to prevent duplicate panels, but (a) it…