Add user setting to specify max number of references in panel

Created by: mrnugget

This is a first step towards fixing #28480 by allowing users to overwrite how many references are shown in the panel.

Also tagging @sourcegraph/frontend-platform here because I touched some components here.

https://user-images.githubusercontent.com/1185253/149158840-76b76c3e-4af2-441c-8507-d23285e8cf54.mp4