R1:82c46f4b93bd

Update "add panel" and "remove panel" Dashboard flows to the new panel storage format

Summary:
Depends on D20407. Ref T13272. This updates the "add panel" (which has two flavors: "add existing" and "create new") and "remove panel" flows to work with the new duplicate-friendly storage format.

- We now modify panels by "panelKey", not by panel PHID, so one dashboard may have multiple copies of the same panel and we can still figure out what's going on.
- We now work with "contextPHID", not…