Feature-flag or revert revamped symbol side bar

Created by: lguychard

Context: https://sourcegraph.slack.com/archives/C03N0HGN069/p1660638973705819?thread_ts=1660045782.489509&cid=C03N0HGN069

What we want to do: put the sidebar behind a second feature-flag, turn it off by default.

/cc @benvenker @lguychard