New reference panel: integrate panel with existing panels

Created by: mrnugget

Right now the experimental reference panel doesn't work well with the existing panels, the history panel & extension contributed panels.

The goal of this ticket is to find a way to allow users to view the history panel, extension panels and the new experimental panel if the feature flag is on. If it's off, we fallback to old panel.

I think the most likely solution (which I've investigated) is to make the new panel one of the builtin panel views here (https://github.com/sourcegraph/sourcegraph/blob/main/client/web/src/repo/blob/panel/BlobPanel.tsx#L152).

Challenges:

• It's the first panel that "listens" to 3+ different actions/IDs: definition, references, implementations_*.
• In a first spike it looks like navigating inside the new panel triggers reloading of extension-contributed panels, leading to new panel names showing up in the list of tabs
• CSS needs to be changed so that we can scroll inside the mini code blob.