Homestyx hydra

R1:f64edb993f68

Allow users who can edit a dashboard to remove invalid / restricted panels

Summary:
Ref T12207. Currently, to remove a panel from a dashboard, it must be a valid panel which you can see.

Instead, only require that the panel PHID actually be listed somewhere in the dashboard's internal list of panels.

This interacts with the "multiple instances of a panel" issue described in some more depth in T12207. In particular:

- Currently, you can sort of add multiple copies of a panel to a dashboard, sometimes?
Repository: R1 hydra
Commit Date: Feb 4 2017