R1:cbe13b3065a1

When editing a tab panel from a dashboard, redirect back to the dashboard

Summary:
Depends on D20396. Ref T13272. Currently, using the dropdowns to edit a tab panel from a dashboard redirects you to the tab panel page.

Instead, redirect back to the context page (usually, a dashboard -- but theoretically a containing tab panel, since you can put tab panels inside tab panels).

Also, fix some JS issues with non-integer panel keys. I've moved panel keys from "0, 1, 2, ..." to having arbitrary keys to make…